(archaic) A small amount, especially a small alcoholic drink.
1917, H. C. McNeile, No Man's Land:
The signal officer was looking wise over something that boomed and buzzed alternately; the machine-gun officer may, or may not, have been enjoying another toothful.
1901, Kate Douglas Wiggin, Penelope's Irish Experiences:
"Give her a toothful of whisky, your ladyship.
1899, Edward Noyes Westcott, David Harum:
"Thank you," said David a minute or two later on, holding out the glass while John poured, "jest a wisdom toothful.
1869, Atticus, Our Churches and Chapels:
Of course it is better late than never, only not much bliss follows late attendance, and hardly a toothful of ecstacy can be obtained in three-quarters of a minute.