In common use since by at least 1654, of uncertain origin. Theories include:
that it derives from rum, or from the last syllable of Latin saccharum (given the harsh taste of earlier rum, the first theory is now considered unlikely),
that it is a shortening of rumbullion or rumbustion, or
that it is from a Romani word for "strong, potent" which is (perhaps) the source of ramboozle and rumfustian (but these drinks were not originally made with rum)