Mouth/maʊθ/ Definition 1 of 6Pro Tip 1/2Facial Expressions The term 'mouth' often helps describe expressions related to emotions or reactions. Her mouth turned into a frown as she heard the news. Pro Tip 2/2Eating and Speaking 'Mouth' is central in phrases about eating and speaking, emphasizing the mouth's function. He talks with his mouth full, which is considered rude.