1620s, "small cup, mug," later of the contents of such a vessel, "small drink" (1690s), a word of unknown origin, possibly related to Norfolk dialectal nog "strong ale." OED considers that the similar Celtic words are "no doubt" from English. Informal meaning "head" is attested by 1866 in American English.
a popular name for a tiny two-winged fly, applied indiscriminately to many small insects, Old English mygg, mycg "gnat," from Proto-Germanic *mugjon (source also of Swedish mygga, Old Saxon muggia, Middle Dutch mugghe, Dutch mug, Old High German mucka, German Mücke "midge, gnat"). No certain cognates beyond Germanic, unless doubtful Armenian mun "gnat" and Albanian mize "gnat" are counted. Watkins, Klein and others suggest an imitative root used for various humming insects and a relationship to Latin musca "fly" (see mosquito). Meaning "diminutive person" is from 1796.
Modern sense of "air-tight vessel of tinned iron" is from 1867. Slang meaning "toilet" is c. 1900, said to be a shortening of piss-can; meaning "buttocks" is from c. 1910, perhaps extended from this.
c. 1200, "a fish-tail," also, apparently, "a person with a fish-tail" (only as a surname), a word of uncertain origin, perhaps from Latin mugil "mullet."
"marijuana, a joint," 1926, apparently originally a New Orleans word, of unknown origin.