Slang: 可不是吃素的 the person is not as weak or nice as you think

可不是吃素的，the literal meaning is "not a vegetarian", but the actual meaning is "the person is not as weak or nice as you think".

e.g.     那家伙可不是吃素的，你提防着点。

           That guy is not easy to deal with, you have to be careful.