A response to someone who obnoxiously complains or cries over something they cannot control. Often said sarcastically, showing that you don't really care about what they have to say.
Alluding to tough onions, which cause you to cry when you try to cut them.
Person 1: ...and then he broke up with me! I cry every night and I don't know what to do with myself!
Person 2: Tough onions.