Green’s Dictionary of Slang

1. to join in, to lend a hand, esp. in a dirty or unpleasant task.

1885		‘’Arry on the Elections’ in Punch 12 Dec. 277/2: Wot’s the good of hus starting the game hup in Town in so proper a way / If them turnip-fed jolterheads muck in, and give Joe the best of the play?
1914		F. Slee diary 3 Oct. 🌐 Many lifelong teetotallers broke the tack and mucked in down the Canteen or Mess.
1917		A.G. Empey Over the Top 48: Six loaves, nineteen men. Let’s see, that’s three in a loaf for fifteen men, — well to make it even, four of you’ll have to muck in on one loaf.
1921		Exter & Plymouth Gaz. 5 May 5/2: Plaintiff said he was told by the Quater-master they must ‘muck in’ together to get the work done.
1937		C.R. Prance Tante Rebella and her Friends (1951) 43: It was a terrible moment when he invited the Predikant to [...] ‘muck in’ as the Settlers say.
1941		G. Kersh They Die with Their Boots Clean 66: Muck in. That’s what the Bible says: muck in. Do unto others as you would ’ave others do unto you. In other words, muck in.
1955		K. Amis letter 28 July in Leader (2000) 440: She is a devout RC, he, a Belfast Prod by origin, mucks in to please her.
1977		(con. 1950s) Nicholson & Smith Spend, Spend, Spend (1978) 49: We’ll all just have to muck in one way or another.
1986		F. Taylor Auf Wiedersehen Pet Two 150: Not me, Dennis – always ready to muck in.
1999		Indep. Rev. 10 July 8: Reminiscing ruefully about his first trips to Africa as an idealistic student, mucking in cheerfully with the locals.
2003		B. Halligan Michael Reeves 10: Staff lived locally or on the site and all mucked in.
2017		S. Bythell Diary of a Bookseller 227: I am fond of her. She pulls her weight. She ‘mucks in’, so to speak.

2. to share, e.g. possessions or living quarters.

1909		Hull Dly Mail 14 Oct. 8/1: We again slept in a big park [...] those that couldn’t ‘muck in’ with anybody had to walk round all night.
1914		Newcastle Jrnl 11 Dec. 3/2: We were what they call in the Army ‘mucking-in-chums’ [...] we used to share our blankets and share any parcels.
1921		N&Q 12 Ser. IX 418: ‘Muck into it’ had a much wider application than the mere sharing of rations.
1929		(con. 1916) F. Manning Her Privates We (1986) 58: Martlow and I have mucked in together.
1965		E. Bond Saved Scene x: Yer can muck in with me a couple o nights.
1978		A. Ayckbourn Joking Apart II i: You can muck in here for a few weeks – no obligation.

3. to have a sexual relationship with.

1938

J. Curtis They Drive by Night 36: He’s known to have mucked in with the girl before he went to prison.