Green’s Dictionary of Slang

1. to swallow carelessly, to eat heartily.

1933		‘William Juniper’ True Drunkard’s Delight 227: Perhaps the invitation was to [...] slosh.
1959		J. Osborne World of Paul Slickey Act I: I stood around sloshing back other people’s champagne.
1965		A. James America’s Homosexual Underground 86: A thousand milling homosexuals, sloshing liquor as fast as they can get it down.
1979		B. Gutcheon New Girls (1982) 133: The rest of the school sloshed through the Wheatena and the marmalade and the toast.
1997		P. Theroux Kowloon Tong 57: She sloshed a mouthful of Milo.
2013		F. Bill Donnybrook [ebook] [S]loshing booze and laying down wagers.

2. to pour out liquid in an abrupt manner.

1875		Chicago Trib. 3 Sept. 2/5: The Ring-paid scribblers and papers will slosh on the usual amount of whitewash .
1914		R. Tressell Ragged Trousered Philanthropists (1955) 191: By the time Harlow and Easton had ‘sloshed’ a lot more whitewash on to them they were mere formless unsightly lumps of plaster.
1952		J.H. Griffin the Devil rides outside 78: [H]e runs with black skirts flying, sloshing water from the pail he carries.
1964		J. Berryman 77 Dream Songs 55: I mentioned fiendish things, he waved them away / and sloshed out a martini.
1975		L.A. Times 24 Feb. 39/1: The French Embassy got the nod about their friends sloshing champagne around on Friday night.
1981		R. Dahl George’s Marvellous Medicine 81: I sloshed in a bit of anti-freeze.