Word Origin & History
1440 (attested as a surname, John Mussheron, from 1327), from Anglo-Fr. musherun, perhaps from L.L. mussirionem (nom. mussirio), though this may as well be borrowed from Fr. Barnhart says "of uncertain origin." Klein calls it "a word of pre-Latin origin, used in the North of France;" OED says it usually
is held to be a derivative of Fr. mousse "moss," and Weekley agrees, saying it is properly "applied to variety which grows in moss." For the final -m he refers to grogram, vellum, venom. Used figuratively for "sudden appearance in full form" from 1590s. The verb meaning "expand or increase rapidly" is first recorded 1903. In ref. to the shape of clouds after explosions, etc., it is attested from 1916, though the actual phrase mushroom cloud does not appear until 1958.