by 1979, initialism (acronym) from role-playing game (see roleplay). As an initialism for rocket-propelled grenade, by 1970.
also jack-pot, "big prize," 1944, from slot machine sense (1932), from now-obsolete poker sense (1881) in reference to antes that begin when no player has a pair of jacks or better; from jack (n.) in the card-playing sense + pot (n.1). Earlier, in criminal slang, it meant "trouble," especially "an arrest" (1902).
The regular Draw-Poker game is usually varied by occasional Jack-Pots, which are played once in so many deals, or when all have refused to play, or when the player deals who holds the buck, a marker placed in the pool with every jack-pot. In a jack-pot each player puts up an equal stake and receives a hand. The pot must then be opened by a player holding a hand of the value of a pair of knaves (jacks) or better. If no player holds so valuable a hand the deal passes and each player adds a small sum to the pot or pool. When the pot is opened the opener does so by putting up any sum he chooses, within the limit, and his companions must pay in the same amount or "drop." They also possess the right to raise the opener. The new cards called for are then dealt and the opener starts the betting, the play proceeding as in the regular game. [Encyclopaedia Britannica, 11th ed., 1911, "Poker." The article notes "Jack-Pots were introduced about 1870."]
To hit the jackpot "be very successful" is from 1938.