Stroppiness is the noun for behaving in a stroppy way: being bad-tempered, quick to take offence, arguing back, sulking, going quiet in an obvious and annoying manner. It is British and informal, and it carries a particular flavour — the person is not furious or dangerous, they are being difficult. Parents say it about teenagers more than about anyone else. A boss might say it about a worker who answers every request with a complaint. Because the word is mild and slightly dismissive, it lets the speaker say 'this is a mood, not a real problem' — which is sometimes fair and sometimes rather unkind, since a person who is repeatedly called stroppy may simply have a complaint nobody wants to hear. It is uncountable: 'a bit of stroppiness', 'all this stroppiness', never 'a stroppiness'. Americans will not usually say it; they might say 'attitude' or 'being difficult'.