a bad habit — a habitual behavior or tendency that is undesirable or harmful.
毎晩遅くまでスマホを見てしまうのが悪い癖だ。
I have the bad habit of looking at my smartphone until late every night.
彼は人の話をさえぎる悪い癖がある。
He has the bad habit of interrupting people when they speak.
飴を食べすぎるのは悪い癖だから控えた方がいい。
Eating too many sweets is a bad habit, so you should cut back.