cheerio, adj. — Green’s Dictionary of Slang

cheerio adj.

[SE cheery, reinforced by cheers!, the toast that precedes that drunkenness]

1. cheerful, merry.

1922		N.Z. Truth 15 Apr. 6/3: Sergeant Shanahan [...] suspecting the latter of having recourse to the ‘cheerio’ stuff.
1923		Wodehouse Inimitable Jeeves 43: I was feeling as cheerio as was possible.

2. (S.Afr.) tipsy, slightly drunk.

1984

Partridge DSUE (8th edn) 203/2: since 1936.