What's the meaning of the phrase 'Be excused'?
Euphemism for 'go to the lavatory'.
What's the origin of the phrase 'Be excused'?
This expression isn't as old as one might imagine. The first use of it in print that I've found is from Harrap's Standard French & English Dictionary, 1947:
Sortir... Est-ce que je peux sortir?, May I be excused?