Other forms: mouthfuls
A mouthful is as much food or drink as you fit in your mouth at one go. A mouthful of hot soup, a mouthful of birthday cake, two mouthfuls left in the bottom of the glass.
It measures roughly, not exactly — the point is one bite or one swallow, so you count in mouthfuls when counting in grams would be silly. The plural is mouthfuls, never 'mouthsful'. The second use has nothing to do with food: a name or phrase that is long and awkward to say is a mouthful, as in her full job title is a bit of a mouthful. English builds a whole family this way — handful, spoonful, armful, roomful — each one meaning as much as the thing will hold.
1nounmouthfulAs much food or drink as fills your mouth at one time.
“He took a mouthful of coffee and pulled a face.”
2nounwordA word or phrase that is long and awkward to say.
“The full name of the department is a bit of a mouthful.”
mouthful is an amount.
Old English already had mūþfull: mouth plus the ending -ful, as much as a mouth holds. The neighbours built the same word out of the same two parts — Dutch mondvol, German Mundvoll, Danish mundfuld, Swedish munfull. It is the sort of word every language would invent anyway, and they all did.
Proto-West Germanic
*munþafull
Old English
*mūþfull
Middle English
mouthful
English, today
mouthful
From Middle English mouthful, mouth-full, mouthe full, from Old English mūþfull, from Proto-West Germanic munþafull, equivalent to mouth + -ful. Compare Dutch mondvol (“mouthful”), German Mundvoll (“mouthful”), Danish mundfuld (“mouthful”), Swedish munfull (“mouthful”), Icelandic munnfylli (“mouthful”). Compare also West Frisian mûlfol (“mouthful”).
Etymology from Wiktionary, CC BY-SA