Monkeyshines are silly tricks, jokes, or naughty play — behaviour that makes you think of a monkey fooling around. The word is almost always plural: people say monkeyshines, not one monkeyshine, in normal talk. It is informal American English, a little old-fashioned and humorous. A parent, a coach, or a newspaper might say “no more monkeyshines” to mean “stop the nonsense.” You would not put it in a serious report. Though actually, it is not only for children: grown-ups can get up to monkeyshines too, and then the word sounds like a mild scolding dressed up as a joke. British speakers may not use it at all.