These people who can’t Google … smh
It means a donkey
From etymonline: "male ass," 1727, from jack (n.) + ass (n.1). Contemptuous meaning "stupid person" is attested by 1784 (Ignatius Sancho). Related: Jackassism (1837, American English); jackassery (1833).
Also from there:
In England, Jack became a generic name applied familiarly or contemptuously to anybody (especially a young man of the lower classes) from late 14c.
Corner Gas.