Green’s Dictionary of Slang

slathered adj.

[? slather v. or dial. slather, to spill, to slobber]

(Aus./US) tipsy, slightly drunk.

1983		Noblesville Ledger (IN) 12 Feb. 3/1: drunk — slathered.
1984		Partridge DSUE (8th edn) 1085/2: since ca. 1920.
2003		McGill Reed Dict. of N.Z. Sl.