excuse oneself
excuse oneself {v. phr.}
1. To think of reasons for not being to blame; think yourself not at fault.
John excused himself for his low grades on the ground that the teacher didn't like him.
2. To ask to be excused after doing something impolite.
John excused himself for his tardiness, saying his watch was wrong.
3. To ask permission to leave a group or place.
The committee meeting lasted so long that Mr. Wilkins excused himself to keep an appointment.
John had to go to the dentist's, so he excused himself and left the classroom.
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Source: A Dictionary of American Idioms