How does 一回だって in this sentence mean "not even once" when there's no negative?

Question

I came across this sentence watching March Comes In Like a Lion. I'll transcribe it to the best of my ability: (here's the episode though, and the quote happens at 5:46)

僕【ぼく】は遠慮【えんりょ】することにばかり気【き】をつけて、実【じつ】は彼女【かのじょ】たちに頼【たよ】られたことって一回【いっかい】だって。そうだ。一回【いっかい】だって。

This is the translation in the subtitles:

I'd been careful not to presume on their kindness, but actually, they've never depended on me for anything. That's right. Not even once.

My best guess is that it's basically an incomplete sentence, with some implied conclusion that makes so much sense / is so common that it just gets dropped? If that's not the case, I just don't get where the "not" and "never" would come from. Any help would be appreciated!

Answer 1

The だって in your example means:

だって 🈩〘副助〙
❹《最小を表す語に付いて、下に打ち消しの語を伴って》全面的否定を表す。…も。
「こんな会社は一日だって我慢できない」 「わずかだってミスは許されない」
(明鏡国語辞典)

Used in this sense, だって(≈も) is always followed by negation. eg

「一回だってない。」「一度もない。」
（*「一回だってある。」 *「一度もある。」 ← incorrect)

And you can see 一回だって/一度も "even once" as a negative polarity item. From the linked answer:

Unlike English, Japanese is verb-final, and negation usually comes toward the end of a sentence. Many Japanese NPIs appear early, and they're often signals that a negative predicate is coming up:

​ 7a. ​ 決して忘れたりしない。
​ 7b. *決して忘れたりする。

In many cases, this signal is strong enough that you can leave out the actual part of the predicate containing the negation (as long as it can be inferred from context): ​

8a. ちっとも変わらない。
​8b. ちっとも。

Similarly, with 一回だって/一度も the predicate containing the negation can often be left out:

「行ったことあったっけ？」　-- 「一度も。」/「一回も。」
"Have you been there before?" -- "No, not even once."
「疑ったことなど、これまでに一度だって！」
"I have never doubted even once!"

Answer 2

See デジタル大辞泉（小学館）

［係助］《断定の助動詞「だ」に係助詞「とて」の付いた「だとて」の音変化という》名詞・副詞、一部の助詞に付く。「でも」に似るが、語調がより強い。

疑問・不定を表す語、または、数量・程度を表す語に付いて、例外なくそうである意を表す。…でも。…も。「だれだって知っている」「一度だって姿を見せない」

だって has multiple meanings. In this usage, there is a strong sense of "even that" where that could refer to an amount, a degree, or anything along those lines. 「一度だってないのだ！」= (literally) there is not even once = Not even once (something has happened)

ない is omitted in that line from the anime, but I am not sure how commonly this kind of omission occurs.