IPA: /ˈmaʊθfəl/
KK: /ˈmaʊθfəl/
The amount of food or other material that can be held in the mouth at one time.
He took a big mouthful of soup and slurped it down.
Mouthful → It is formed from "mouth" (from Old English *muð*, meaning the opening of the face used for eating and speaking) and "-ful" (meaning full of). The word "mouthful" refers to a quantity that fills the mouth.
Think of a 'mouth' being 'full' — that's why a mouthful is a quantity that fills the mouth.