1Especially of a container or receptacle: having a wide opening.
2Of a person or animal: having a wide mouth.
3Having the mouth wide open, or opening the mouth wide, in order to eat or swallow; (hence) voracious, devouring, destructive.
4Having the mouth wide open, or opening the mouth wide, in order to speak or to express an emotion (as amazement, surprise, etc.); characterized by having the mouth open in such a manner. Hence also: loud, unrestrained; (in early use) †using long words or inflated language (obsolete). Of a person.
5Of a person's verbal or facial expression, emotion, etc.
Mid 16th century; earliest use found in Thomas Becon (1512/13–1567), theologian and Church of England clergyman. From wide + mouthed.