In Italian, 'puntiglio' means "a fine point," hence a verbal quibble, and is most likely the source of the English "punctilious."
A pun is defined by Webster as "the humorous use of a word, or of words which are formed or sounded alike but have different meanings, in such a way as to play on two or more of the possible applications; a play on words."
From www.punoftheday.com: Link
Why do people groan when a pun is told? A pun is often considered obvious humor, since the person relating it is merely balancing the humor in it on a twist of a word's meaning or sound. Children love this type of obvious humor and can laugh at it without reproachments. Adults, on the other hand, are more likely to have a twinge of envy, and "why didn't I think of that?". It is this envy in adults that subconciously causes them to groan upon hearing a pun. As time goes on, it can only be hoped that we adults will eventually learn to react more like a child and less like a groan-up!
Come on, kids... show me whatcha got! You know what they say: "seven days without a pun makes one weak"... so what's your favorite punny phrase?