low-blow

This it the term given to describe a very deep insult that could also be obvious to most.

e.g. Mike's mum recently died due to cancer and Brad goes too far with an insult.

Mike-"Lol you'll never be as cool as Hagelberg!"

Brad-"Ha! you're such a bitch, even more so now that your mum's dead!"

Mike-"Go fuck yourself! That was a low-blow Brad!"