by 1979, initialism (acronym) from role-playing game (see roleplay). As an initialism for rocket-propelled grenade, by 1970.
1590s, from Russian beluga, literally "great white," from belo- "white" (from PIE *bhel-o-, suffixed form of root *bhel- (1) "to shine, flash, burn," also "shining white") + augmentative suffix -uga. Originally the great white sturgeon, found in the Caspian and Black seas; later (1817) the popular name for the small white whale (Delphinapterus leucas) found in northern seas.
edible cruciform plant (Eruca sativa) used originally in the Mediterranean region as a salad, 1967, the American English and Australian form of the name (via Italian immigrants), from a dialectal variant of Italian ruchetta, a diminutive form of ruca-, from Latin eruca, a name of some cabbage-like plant (from PIE *gher(s)-uka-, from root *ghers- "to bristle;" for which see horror).
In England, the usual name is rocket (see rocket (n.1)), which is from Italian ruchetta via French roquette. It also sometimes is called hedge mustard.