What would be what is not understood in 'brushing the teeth'? Perhaps as an irony, when it is used by the absurd and in truth something else is being said; but these meanings are interpreted by context rather than by definition.
BRUSH YOUR TEETH Textually, brush your teeth with an ad hoc bristle brush. Figuratively it is used to denote repudiation, sarcasm regarding something said earlier by the interlocutor: Go brush your teeth with your vaccine!