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:
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?