Common phrases you might hear:
"he got pwnd" translation: he got beat really bad.
"that is agg, he is agg, etc" translation: that is cool, he is cool
"This headband is legit" translation: can mean it's "cool" or can mean that it is a genuine old headband and not a recently made copy.
"OG" translation: cool and oldschool at the same time
"I mowed those muppets last game" translation: "I owned (see above) those noobies, typically involves some overshooting/bonus balling"
"that gun shoots ropes, lasers, sex, etc" translation: "that gun shoots extremely nice and fast (BPS)"
Thats all I can think of for now.
O, and the one thing that I returned to the sport to find going on that I really hate is people making up names for their guns based on colors. Example: A blue and red Ego is no longer a blue and red Ego, but rather it is referred to as "Superman Ego." I think this is used to make it seem like they have a super rare custom gun when really its just a couple swapped parts.