A balls-up is something done very badly — a mess, a failure, a job handled in a clumsy way. It is informal British English and a bit rude. You might hear an adult say “What a balls-up.” Americans usually say screw-up or mess-up instead. There is also a verb: to balls something up, meaning to do it badly, and people sometimes say go balls up. Though actually, you will also see it written balls up as two words; the meaning does not change. It is not a word for an essay or for talking to a teacher.