also Mick, derogatory slang for "an Irishman," by 1856, from the nickname form of the common Irish given name Michael (q.v.). Micky is attested in U.S. slang for "an Irish boy or man" by 1858.
cartoon mouse character created 1928 by U.S. animator Walt Disney (1901-1966). As an adjective meaning "small and worthless, petty, inconsequential" it was in use by 1951, perhaps from the popularity of the cheaply made Mickey Mouse wristwatch; it was used by 1935 in reference to mediocre dance-band music, based on the type of tunes played as background in cartoon films.
"drink laced with chloral hydrate," by 1918. Mickey Finn was used from the 1880s as the name of the main character in a series of popular humorous Irish-American stories published by New York Sun staff writer Ernest Jarrold (1848-1912), who sometimes also used it as his pen-name. Perhaps there is a connection.
"great, large; much, abundant; a great deal," a dialectal survival of Old English micel, mycel "great, intense, big, long, much, many," from Proto-Germanic *mekilaz (source also of Old Saxon mikil, Old Norse mikill, Old High German mihhil, Gothic mikils), from PIE root *meg- "great." Its main modern form is much (q.v.); the common Middle English form was muchel. The phonetic development of the dialectal survival is obscure and might reflect Old Norse influence. Related: Mickleness. Middle English had muchel-what (pron.) "many various things."