Green’s Dictionary of Slang

a snack, a meal, a drink.

1918		‘Commander’ Clear the Decks! 64: One man stand by on each. The rest of you in for a mug-up and dry clothes.
1930		(con. 1910s) L. Nason A Corporal Once 113: There was a hook-pot at the other end of the grating, so that the watch could have a mug-up.
1931		B. Lubbock Bully Hayes 100: Old Luke, who had quietly slipped into the cabin for what Blue Nose fishermen [...] call a ‘mug up’.
1942		Amer. Neptune July 236: The crew [...] pile down into the forecastle for their sunrise ‘mug-up’ [HDAS].
1957		Sat. Eve. Post 16 Mar. 139: At our noon mug-up at the cabin I heard a piece of news [HDAS].
1972		Cleaves Sea Fever 246: I went to the p.o. mess to get a mug-up of coffee.