to insult someone on an elevated level; to put someone down to the lowest; usual victims of a "sick burn" will be the guy in your school who tucks his plaid shirt into his pants and clips a cell phone on his belt
a good way to know if you've been sick burned is that whoever has burned you will immidiatly shape his hands like two guns and immitate blowing the smoke off them on at a time and will say in a baratone voice : "sick burn", possibly with a clever smirk on his face.. you must have a mustache and a red leather jacket to sick burn someone
Bobby: "Hey Jimmy, wanna go for a ride"
Jimmy: "Yeah,.... on your MOOOMMMM.."
Jimmy: "Oh.... sick burn"