In the pilot episode of Friends Monica meets this handsome guy and they have sex in her flat. Next day in the morning, she kisses him off and then she has this conversation with Rachel:
Monica: So how're you doing today? You-you sleep ok? You talked to Barry? I can't stop smiling.
Rachel: I can see that. You look like you slept with a hanger in your mouth.
Monica: I know, he, he's just so... You remember you and Tony DeMarco?
Rachel Oh, yeah.
Monica: Well, it's like that. With feelings
Rachel: Oh wow. Are you in trouble.
Monica: Big time!
I have no idea what hanger means here and how it is related to her smiling.