Green’s Dictionary of Slang

a popular toast when drinking.

1861		R.F. Burton City of the Saints 545: We acknowledged his civility with a ‘here’s how,’ and drank Kentucky-fashion.
1902		O. Wister Virginian 118: ‘Here’s how!’ said he [...] And they pledged each other in the tin cups.
1920		‘To Hear Him Tell It’ in J.A. Lomax Songs of the Cattle Trail 34: ‘So let’s drink up; here’s how!’ / As we drain our glasses two.
c.1935		‘Betty Boop in “Flesh”’ [comic strip] in B. Adelman Tijuana Bibles (1997) 28: Here’s How.
1942		A.L. Haskell Waltzing Matilda 38: ‘’Ere’s ’ow!’ ‘Skin-orf-yer-nose!’.
1970		E. Thompson Garden of Sand (1981) 137: ‘Here’s how.’ She lifted the water tumbler and knocked back the fiery whiskey.
2000		(con. 1940s–60s) Décharné Straight from the Fridge Dad.