society > morality > duty or obligation > moral or legal constraint > immunity or exemption from liability > justification > justify [verb (intransitive)] > have sufficient excuse or be exculpated (2)
intransitive for reflexive. Obsolete. rare.
to be excused: to be allowed to leave a room, esp. a schoolroom; spec. to go to the lavatory. colloquial (chiefly in school language).