Bad behavior; mischief. A colloquial term for wrongdoing, pranks, or things that cause trouble.
子どもが庭で悪さをして花を折ってしまった。
The child misbehaved in the yard and broke the flowers.
パソコンに悪さをするウイルスを検出したので、すぐに削除した。
A virus that harms the computer was detected, so I deleted it immediately.
あの猫は時々悪さをするが、憎めない性格だ。
That cat sometimes gets up to mischief, but you can't dislike it.