Cheap shot means someone took advantage of you or your situation by making fun of you. When Mark called Karen fat after she got pregnant, he took a cheap shot. hope that helps.

in this expression cheap means unfair, underhanded shot means hitting another person in some way (literally or figuratively, such as a verbal attack) The referee blew the ball dead, so John relaxed. Then Mike hit him, it was a cheap shot. Sue: I think we should send more aid to Haiti. Jill: I can see why you would think that, since you dropped out of college! Here Jill took a cheap shot at Sue. Instead of answering Sue's argument with a counter-argument, she responded with a personal insult, which contributed nothing to understanding whether more aid would be appropriate or not. http://www.merriam-webster.com/dictionary/cheap%20shot