ウソついたら、オオヤマフグリのトゲ、百万本の～ましゅ！
I know it's pinky swear - "If you lie, you'll swallow million thorns!"
But what is オオヤマフグリ?
Is it an imaginary thing, maybe 大山陰嚢, something like "high mountain pinecone"?
Edit: It's from video game "Xenogears". A dialogue with a little girl in the item shop.
ほんとの、ほんとね？
そいじゃ、ゆびきり！
ウソついたら、
オオヤマフグリのトゲ、
百万本の～ましゅ！
これで、オッケー！
ふたりの燃えるハートは、
かちゃいきずなで結ばれたのよ！
ウソついたら、オオヤマフグリのトゲ、百万本の～ましゅ！
As for オオヤマフグリ, it must be typo for オオイヌノフグリ, whose meaning is explained in Igor Skochinsky's answer.
"百万本の～ましゅ" sounds like what is pronounced by a toddler, and is an euphonic change from "百万本[飲]{の}ます". So your interpretation as "If you lie, you'll swallow million thorns!" is partially incorrecrt: it should be like "If you lie, I'll make you swallow million thorns!"
I guess it's a pinky swear, but we rarely use the phrase for a pinky swear; To tell you the truth I've never heard it.
We use one of the following phrases to make a pinky swear:
指切｛ゆびきり｝り means to cut your fingers.
拳万{げんまん} means to punch you hard ten thousand times.
針｛はり｝千本｛せんぼん｝飲｛の｝ます means, as you know, make you swallow a thousand needles
I didn't know what "pinky fingers" means, so I look it up on the Internet. It's very interesting to know that we also do the same gesture with joining pinky fingers in a symbolic gesture made when children swear on a promise together.
This seems to be an exported culture/custom from Japan.
Thanks to goldbrick's comment, I re-examined.
On second thought, オオヤマフグリ is a fictional plant based on オオイヌノフグリ and it has an incredibly large number of sharp thorns.
As is written in Yuuichi Tam's answer, the author who created the name "オオヤマフグリ" seems to have a very rich imagination. So it is no wonder he thought of the name from what is beyond our imagination. With knowing the above things, I'll think about what the name came from.
It is hard to assume that the author thought of "オオヤマフグリ" from "オオイヌノフグリ" when thinking again now. The reason is that if you search for the word "オオイヌノフグリ" on the Internet, you could soon understand that most of the Japanese people do not know or do not use it because there are only seven hits. Besides, it is a pretty flower that doesn't have such horrible thorns at all.
Next, the word "ふぐり" or "フグリ" means testicles in English, and the total number of hits is 422700, so the possibility that the author knew it was strong, but it is hard to understand that the author quoted it as part of the name accompanied by the feeling difficult to use forcibly.
Well then, what did the author get the wording of "オオヤマフグリ" from? If you think it sincerely, you have to swallow it when you tell a lie or break your promise, so you have to start with thinking that it should be plants or animals having thorns, burrs, spines or quills; or [針]｛はり｝ (spines, quills, lit. needle), [棘]{トゲ} (thorns, splinters, spines) or [毬]{イガ} (burrs) .
Talking of "spines or 針｛ハリ｝", we soon image a "hedgehog or 針鼠｛ハリネズミ｝ (lit. needle-rat or spine-rat)" that is a spiny mammal. However, since hedgehog is a small and cute mammal, and its spines are short, it doesn't have an enough image to threaten a liar with making him/her swallow its spines. Resembling a hedgehog, we could soon image a "porcupine or山荒{ヤマアラシ} (lit. mountain-devastator or mountain-buster) with a coat of long and sharp spines. Spines of a porcupine have a sufficient image of threatening a liar. From the Japanese name of "ハリネズミ or hedgehog", we come up with a fish with many spines called "針千本{ハリセンボン} or porcupinefish". According to Wikipedia, porcupinefish have the ability to inflate their bodies by swallowing water or air, thereby becoming round. This increase in size (almost double vertically) reduces the range of potential predators to those with much bigger mouths. A second defense mechanism is provided by the sharp spines, which radiate outwards when the fish is inflated. But, the pronunciation of "ハリネズミ or hedgehog", "ハリセンボン or porcupinefish" or "ヤマアラシ or porcupine" is too much different from "オオヤマフグリ" whose origin we are looking for.
"イガ or burrs" means the skin of a plant full of thorns, so イガ satisfies the first condition as a candidate for オオヤマフグリ.
Talking about "イガ or burrs", "毬栗｛イガグリ｝ or chestnut" is famous, which is a Japanese chest nut lit. chestnut in burrs. When I was looking into "イガグリ" on the Internet, I found an explanation that イガグリ which grows naturally is called "柴栗｛シバグリ｝" or "山栗｛ヤマグリ｝". "This is it!" I thought. If "山栗｛ヤマグリ｝ lit. mountain chestnut" grows deeply in big mountains, it could be called "大山｛オオヤマ｝栗｛グリ｝" . Don't you think "オオヤマグリ" sounds like "オオヤマフグリ"? I will decide that "オオヤマグリ" is the origin of "オオヤマフグリ".
Let's wait for the correct announcement from the author.
Yuuichi Tamさんの回答にあるように、オオヤマフグリという名前を創造した作者は発想が大変豊かな方だと想像します。従って我々の想像を超えたところから名前を考えたとしても全く不思議ではありません。それを承知で考えてみました。
今あらためて考えますと、「オオイヌノフグリ」から「オオヤマフグリ」を考えたとは想定しにくいですね。その理由は、インターネットで「オオイヌノフグリ」という言葉を検索しますと、ヒット数がたった７件しかないほど日本人のほとんどが知らないあるいは使わない単語だからです。しかも、恐ろしいほどのトゲやハリがない可憐な花です。
次に、「フグリ」「ふぐり」と言う言葉は英語ではtesticles あるいは ballsという意味で、ヒット数が合計して422700件あるので、作者が知っていた可能性は高いですが、名称の一部として敢えて使うのには遠慮が伴う言葉であり、作者が参考にしたとは考えにくい気がします。 なお、「フグリ」は、ポケットモンスターリーフグリーンの女主人公にファンが付けたあだ名として有名であるとの記事が見つかりましたので、アニメ業界では比較的知られた言葉とは推察できます。あだ名としての「フグリ」は、「リーフグリーン leaf green」の中間の発音から造った呼称のようですが、本来の「フグリ testicles」が持っている発音と同じことが分かり、一旦意味が分かってしまうと、使うのが憚（はばか）られ、今ではこの女主人公を「リーフ」と呼ぶようになったようです。大変常識的な解決方法だと思います。 「ふぐり」ついでですが、誰もが知っている「松ぼっくり」も語源的には「松ふぐり」のようです。しかし、「松ふぐり」もヒット数が800数十ですので、死語に近いと思われ作者が参考にしたとは考えにくいです。
それでは、一体どこから「オオヤマフグリ」を考えついたのでしょうか。 素直に考えますと、嘘をついた人に飲ますのですから、「針」や鋭い「トゲ」がいっぱいあるもの（動植物）が連想のスタートでしょう。 トゲ、ハリ、イガなどからは、「ハリセンボン」「ハリネズミ」「イガ栗」「イバラ」などが思い浮かびます。「ハリネズミ」は少し優しく可愛い動物なので、その連想から外国の「ヤマアラシ」が浮かびます。本当に痛そうなトゲがいっぱいあります。それでも、いずれも「オオヤマフグリ」と発音が違い過ぎます。
「イガ」とは棘（トゲ）のいっぱい生えた植物の皮を指すようですので候補として合格です。「イガ栗」を調べておりますと、自生するものは、別名「柴栗」「山栗」と言うとあります。これだ！と思いました。「ヤマグリ」。山深く大山に自生している山栗、「オオヤマグリ」です。似ていますね。「オオヤマフグリ」と「オオヤマグリ」。これで決まりとします。あとは、作者からの正解発表を待ちましょう。
It seems to be a plant name (possibly a very thorny one). I could not find this specific one but Wikipedia has オオイヌノフグリ (Veronica persica) which may be related. I did find a blog post which has a picture mentioning オオヤマノフグリ: https://lineblog.me/maze8074/archives/345233.html
I found this comment "イチゴカブト リンゴユリ カミナリダイコン・・ってまあ実に多彩なネーミングです。これは植物に限らず動物も然り。牛はクマギュウですし、トンガリイボザルやらオオヤマフグリやらゾウツバメやら、なんか思わず「一体どんな動物なんや？」なんて想像しちゃいそうですね（笑）他にもモンスターのネーミングとか、ゼノはなんか「ネジが一本抜けたような」センスで思わずにやりとしちゃいます。(There are various fictitious names like Ichigokabuto, Ringoyuri and Kaminaridaikon. They are not only for plants , but also for animals like a caw is called Kumagyu, other animals are called Tongariibozaru, Ooyamafuguri and Zoutubame. We may wonder how animals they are, when we hear their fictitious animals' names. Monsters'names are also odd. The names in the game of Xenogears are odd and they make me smirk." in internet.
He seemed to talk about the characters of the video game "Xenogears". So I think オオヤマフグリ is a character of the game which is like porcupines.
I can't find the description that オオイヌノフグリ has とげ, the spikes. Therefore, オオイヌノフグリのトゲ doesn't make sense.
I think maybe the writer confused オオイヌノフグリ and ヤマアラシ.
ヤマアラシ is "hedgehog."
However, it is quite absurd to confuse オオイヌノフグリ, a plant, with ヤマアラシ, an animal.
In short, I don't understand the sentence. Maybe the writer was talking about their own imaginary creature.
