Green’s Dictionary of Slang

baptize v.

also baptise
[i.e. to ‘immerse in water’]

to dilute wine or alcohol.

implied in baptized
[UK]Grose Classical Dict. of the Vulgar Tongue.
[US]Ersine Und. and Prison Sl.

In derivatives

baptized (adj.)

1. of alcohol, usu. spirits, watered down.

J. Healey Theophrastus 46: He will give his best friends his baptised wine [F&H].
[UK]Grose Classical Dict. of the Vulgar Tongue n.p.: Baptized, or christened. Rum, brandy, or any other spirits, that have been lowered with water.
[UK]Lex. Balatronicum.
[UK]B.M. Carew Life and Adventures.
[UK]‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc.
[UK]Duncombe New and Improved Flash Dict.
[US]Matsell Vocabulum 10: baptized Liquor that has been watered.
[Aus]Crowe Aus. Sl. Dict. 6: Baptised, [...] water in liquor.
[US]Monteleone Criminal Sl. (rev. edn) 17: baptized booze Cut liquor.

2. (Aus.) drowned.

[Aus]Crowe Aus. Sl. Dict. 6: Baptised, drowned.
[Aus]N. Pulliam I Travelled a Lonely Land (1957) 230/1: baptized – drowned.