Well Omar, a shot of Vodka at a bar that`s cheap, is also a cheap shot I guess. Sounds logical to me. I think there`s another type of „cheap shot“ I might have heard once. Say, you`re hunting deer and the deer gets really close and you blow its head off (poor deer) and it was too easy, someone could say, „Man, that was a cheap shot. That deer didn`t have a chance.“ 

Another use of cheap shot. 

@Laura

That was kind of a cheap shot trying to make everyone guess what a cheap shot is. Of course, it could also be a low-priced shot of whiskey at a bar. 

Ok ..

so a cheap shot is a mean remark or action,  that takes unfair advantage of a known weakness of the target. (Mirriam-Webster dictionary)

cheap - mean it wasn't worthy of the person, undignified

shot - an insult thrown at someone.

"I thought the remark about her weight was a cheap shot"