IPA: /ˈtʌmbərfəl/
KK: /ˈtʌmbərfəl/
A quantity that a tumbler can hold, often used to measure drinks.
He poured a tumblerful of water into the glass.
The word 'tumblerful' is formed from 'tumbler' (from Middle English 'tumblere', meaning a drinking vessel) and '-ful' (meaning full of). The word 'tumblerful' means full of tumblers or a quantity that fills tumblers.
Think of a 'tumbler' as a drinking vessel and remember that 'tumblerful' means something that is full of tumblers.