Green’s Dictionary of Slang

shant v.

also shant it up
[shant n. (2)]

to have a drink.

[UK]J. Sullivan ‘Ashes to Ashes’ Only Fools and Horses [TV script] Oi, don’t you think this is the wrong time and place to be shanting it up?
[UK]G. Tremlett Little Legs 197: shant to have a drink, especially Scotch.