Meaning "Random Bullshit Luck". Originating from the game 'Counter-Strike' after a Professional Player became furious after receiving multiple headshot deaths from an 'RBSL' player who just started playing. Examples of RBSL would be players who 'spray' and randomly kill enemies with headshots.
"Dude chase is so RBSL, all he does is spray!!"
"wow that new kid is way to fucking RBSL.. he sprayed the shit outta me"
"dude she just got lated by that RBSL NIGGA GREG"