Green’s Dictionary of Slang

1. a missionary.

1939

J. Cary Mister Johnson 164: That’s wot’s wrong with some of these teetotal mishes — they get fat on the brain .

2. in sexual intercourse, missionary position.

1997		A. Petkovich X Factory 195: A white guy in his early forties wearing a Gang Bang 2 T-shirt is soon fucking Jaz in the mish pazzish.
2000		‘Sugar 3’ Mr Web Rev. Aug. 🌐 They move into doggy & Marcus slams her like there isn’t a tomorrow, but she seems too much for him. They switch to missionary and Marcus stops every few strokes to keep from cumming but once they switch to spoon its more than he can take, he slips in one quick mish then let’s loose the juice.

3. (N.Z.) hard work, in the sense of a ‘mission’.

2003

McGill Reed Dict. of N.Z. Sl. 135: mish Hard work, an abbreviation of ‘mission’ in the sense of something being a mission.