Mouthful

wiktionary

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From Middle English mouthful, mouth-full, mouthe full, 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”).

mouth +‎  -ful. 

etymonline

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mouthful (n.)

early 15c., "as much as a mouth can hold," from mouth (n.) + -ful. Meaning "a lot to say" is from 1748.