Green’s Dictionary of Slang

slopping up n.

[slop n.1 (1d)]

(US tramp) a session of heavy drinking.

[US]J. Flynt Tramping with Tramps 271: The bums intended to have a great ‘sloppin’-up’ (drinking-bout).
[US]P. & T. Casey Gay-cat 304: Slopping-Up—getting a big drunk; as a noun, a drunk.