Green’s Dictionary of Slang

lap n.2

[SE lap, to drink]
(UK Und.)

1. [mid-16C–mid-19C] buttermilk or whey or any thin, non-alcoholic drink.

2. [early 17C–1930s] liquor in general.

3. [mid-17C–mid-19C] soup.

4. [early 18C–1900s] (also brown lap) tea.

In compounds

lap-feeder (n.)

[late 18C–1900s] a silver tablespoon.

In phrases

go on the lap (v.)

[late 19C–1900s] to go out drinking.