mouth

Concise Oxford English Dictionary © 2008 Oxford University Press:
mouth/maʊθ/

▶noun (pl. mouths)

• 1 the opening and cavity in the lower part of the face, surrounded by the lips, through which food and air are taken and vocal sounds are emitted.

  ■ the corresponding opening through which an animal takes in food.

• 2 an opening or entrance of a hollow, concave, or similar structure.

  ■ the opening of a container.

• 3 the opening or entrance to a harbour or bay.

  ■ the place where a river enters the sea.

• 4 informal talkativeness or impudence.

▶verb

• 1 move the lips as if to form (words): she mouthed a silent farewell.

  ■ say in an insincere or pompous way.

  ■ (mouth off) informal talk in an opinionated or boastful way.

• 2 take in or touch with the mouth.

– phrases
be all mouth (and no trousers) informal tend to talk boastfully but not to act on one's words.
give mouth (of a dog) bark.
keep one's mouth shut informal say nothing; avoid revealing a secret.
watch one's mouth informal be careful about what one says.

– derivatives
-mouthed adjective,
mouther /ˈmaʊðə/ noun,
mouthless adjective.

– origin OE mūth, of Gmc origin.