Originates from TF2 and is the response someone gives when killed by a critical hit.
Critical hits are random in TF2 and usually causes one hit kills, prompting players suddenly killed with them to respond with "lolcrits."
1v1 Soldier Fight
Soldier A hits Soldier B twice with rockets
Soldier B fires a critical rocket at soldier A, resulting in an instakill
Soldier A: lolcrits