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.
Middle English mous, from Old English mus "small rodent," also "muscle of the arm" (compare muscle (n.)); from Proto-Germanic *mus (source also of Old Norse, Old Frisian, Middle Dutch, Danish, Swedish mus, Dutch muis, German Maus "mouse"), from PIE *mus-, the old Indo-European name of the mouse, retained in several language families (source also of Sanskrit mus "mouse, rat," Old Persian mush "mouse," Old Church Slavonic mysu, Latin mus, Lithuanian muse "mouse," Greek mys "mouse, muscle").
Plural form mice (Old English mys) shows effects of i-mutation. As a type of something timid or weak, from late 14c. Contrasted with man (n.) from 1620s (nor man nor mouse). Meaning "black eye" (or other discolored lump on the body) is from 1842. Computer sense of "small device moved by the hand over a flat surface to maneuver a cursor or arrow on a display screen" is from 1965, though the word was applied to other things resembling a mouse in shape since 1750, mainly in nautical use.
Parturient montes, nascetur ridiculus mus [Horace]
"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.
"resembling a mouse or rat," c. 1600, from Latin murinus "of a mouse," from mus "mouse" (see mouse (n.)).