Mick

google

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early 20th century: of unknown origin.

wiktionary

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From a common Irish name Michael

Abbreviation of Mickey Mouse.

etymonline

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mick (n.)

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.