A "c2" is a word used for a slut or permiscious girl. Or when used for guys it means a threat. It was derived from counter strike source when if the button c then 2 was pressed you would tell your teammates "enemy spoted"
When refering to girls: "Oi bro check out that c2 there. Damn shes hot."
When refering to guys: "Check out the c2s that jus walked in."
The use for girls is the most common.