Green’s Dictionary of Slang

1. (orig. Aus.) drunk.

1910		L. Esson Woman Tamer in Ballades of Old Bohemia (1980) 71: I bring you in all I get, don’t I? I don’t get lathered on the takings, do I?
1927		‘Sl. Expressions for Drunk’ in New Republic in AS XVI:1 (1941) 9 Mar. 70: [...] lathered.
1949		Wodehouse Mating Season 29: So you were lathered last night?
1991		D. Jenkins You Gotta Play Hurt 100: ‘We’ll creep some brews and get lathered’.

2. (US drugs) intoxicated by a drug.

1959

J.E. Schmidt Narcotics Lingo and Lore.

3. (US) sexually excited.

1974

C. Loken Come Monday Morning 63: Lookit the way she’s wigglin’ around [...] she was probably all lathered up already.