Frag is a gaming term that refers to a successful kill in a first-person shooter (FPS) game. The term was popularized through multiplayer death match modes, where players compete in an arena and try to rack up the most kills. The term frag may also refer to killing a player using a fragmentation grenade (frag grenade).
Techopedia explains Frag
Like many video game terms, the origins of the term frag are not clear. Frag may be an adaptation of the term fragging, which is a criminal act where a military serviceman kills a superior officer using a grenade. Or it might be a (slightly) more innocent reference to how many characters could be blown into small fragments by the right weapon. Frag can be used as a verb as in “I love fragging noobs” or “that guy fragged me before I was ready.”