Webster's Revised Unabridged Dictionary · 1913

frag

frag (frag) , transitive verb

[by shortening from fragmentation grenade; ca. 1965.]

This term became popularized when disaffected American troops serving in Vietnam were reported on occasion to kill unpopular commanders by the use of grenades.

(Military) To assault, especially to kill or wound, with a fragmentation grenade. [Slang]