Lad is another word for a boy or a young man. A man who's been hit with a water balloon might turn around to a group of giggling boys and ask, "All right, which of you lads did that?"
You can use the casual lad instead of guy, fellow, or chap. This word is much more common in Britain than in the U.S., although everyone understands what it means. Someone in London might talk about going out on the town with "the lads," while in Boston people are more likely to say they're meeting "the guys." Lad, originally spelled ladde, first meant "foot soldier," or "young servant."
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