С дураками не шутят

It is not worth bantering a stupid man as humo(u)r is beyond his comprehension, and he may get angry

Var.: С дураками шутить опасно Cf: Jest with an ass and he will flap you in the face with his tail (Br.). Jest with an ass and he will slap you in the face with his tail (Am.). Never joke with a fool (Am.)