When I was at school and someone had a mineral, the main way to ask for a drink from it was to ask for a slug.
Now this didn't mean I'd gone all French and wanted a gastropod mollusk to go with my drink ! Perish the thought. No it just meant I wanted a drink from a bottle or can of soda.
Slug : a drink, a sip, a slurp.
"Gizza wee slug o' your mineral." The usual response to this was "feck off" but it never hurt to ask.
The great thing about a slug was that it was of indeterminate size. You could take a wee drink, swallow it and take another. A slug usually meant as many repeats of this as the owner of the drink would permit before he wised up and snatched it back.
Of course, thinking back now, it may have just been me being greedy !
That's always a possibility.