Green’s Dictionary of Slang

1. a lump of expectorated phlegm.

1970		Current Sl. V (Winter) 8: Hang a louie, v. To spit on someone [HDAS].
1986		S. King It (1987) 911: She drew in a great, hitching breath and hocked a remarkably large looey onto the top of his head.
1990		G.R. Clark Words of Vietnam War 288: Louies...GI slang for ‘spit,’...especially that less than clear variety [HDAS].

2. a piece of nasal mucus.

1980

National Lampoon Aug. 92: Jimmy Picks! It’s Real! Carter gets caught reaching for a Louie!