Secret or dishonest behaviour.
“Oh there’s Billy, up to his old shenanigans again.”
Usually it’s mischievous behaviour so whatever you consider that to be could be considered “shenanigans.”
Stealing the neighbour’s cat for a week so your neighbour has to put up “missing cat” posters would be considered shenanigans. Dropping the cat off in the backyard without being seen the next week and then stealing the cat the second week (so you steal the cat every other week) would be considered abuse but also “shenanigans.”
Who’s been up to any “shenanigans” this week? Or do you want to admit it?