Could I get help with this translation please?

Question

I've been learning on my own for about half a year and I'm translating a drama CD - so going only by hearing, I don't actually have the transcript. Here is what I'm hearing:

子供でもあるまいに、あんなにはっしゃいで、困ったやつ。

Context: Two gentlemen had a short conversation about food and decided to go to the cafeteria, one of them excitedly ran off as soon as they had made the decision, this is the other, slowly following, commenting on the first gentleman's departure.

Problem 1: Could I please get a clear explanation of how the でもあるまいに works? I gather that it's something along the lines of "even though [he's not a child]", but I'm not so sure about the nitty-gritty of the actual usage.

Problem 2 (probably stupid, sorry): I'm not exactly sure whether I have the middle part down correctly, specifically the はっしゃいで part. I've seen it translated by a trusted source as "and yet he's running so quickly" but can't match up what I'm hearing to this meaning at my level (the character generally tends to use very polite language which often complicates things for a beginner like me). So if anybody has an idea what the word/grammar could be a would please explain this to me? I know that's a weird question (since I can't provide the audio), but if somebody would be willing to make a guess here, please?

Answer 1

First of all, the verb is 「はしゃぐ」 and not 「はっしゃぐ」. Accordingly, the te-form is 「はしゃいで」 with no small っ in it.

Thus, the line being uttered would presumably be:

「子供{こども}でもあるまいに、あんなにはしゃいで、困{こま}ったやつ。」

That is indeed a perfectly grammatical and natural-sounding sentence that makes sense. Needless to say, a word like 「だ」 is left unsaid at the end, but that is also quite normal.

「あるまいに」 is a somewhat literary way of saying 「ないのだから」 or 「ないのに」. 「まい」＝「ない」 in meaning. You will keep encountering 「まい」, trust me.

I've seen it translated by a trusted source as "and yet he's running so quickly"

Why is it trusted? 「はしゃぐ」 does not mean that, at least not directly.

「はしゃぐ」 means "to whoop it up", "to make merry", etc. Running is completely optional. Isn't this what one of the guys is doing?

「あんなにはしゃいで」 means "whooping it up like that and..." Here, "like that" would mean "like crazy".

So, I might translate the line to:

"Though he surely isn't a kid, he's whooping it up like crazy. He's such a baby."