Green’s Dictionary of Slang

1. a drinking bout, a drunken spree; usu. as on the mop.

c.1860		Newspaper Cutting in Farmer & Henley (1890–1904) He’d been having a mop, as he called it, because he was on piecework.
1873		Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.

2. a drunkard.

1873		Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.

3. a drink, alcohol.

1901

D. Cotsford Society Snapshots 113: Sir Startin Price (invitingly) More ‘mop’? Lady de Handicap (approvingly) Good idea.