(even) a worm will turn
Even quiet, submissive people will get angry if you continually agitate them. Sweet little Janine is going to snap at you if you keep bugging her—even a worm will turn sometimes.
Even a worm will turn.
Prov. Even a meek person will become angry if you abuse him or her too much. You'd better stop maltreating Amy. She's a mild-mannered woman, but even a worm will turn.