Green’s Dictionary of Slang

a communal activity.

1914		T. Hampson diary 8–17 Aug. 🌐 I got an occasional decent meal in the town, but it was mostly what became known as ‘mucking in’.
1933		(con. WWI) F. Richards Old Soldiers Never Die (1964) 256: The bank clerk, architect and Sealyham had also arrived back and we all had a muck in.