Not good enough to be allowed or tolerated.
“Waiting three weeks for a response is simply unacceptable.”
“The condition of the rental car upon pickup was unacceptable.”
Use 'unacceptable' in CELPIP Writing Task 1 complaint emails to firmly but politely state that a situation must change, such as 'This delay is unacceptable.' It adds emphasis without sounding rude, as long as you follow it with a specific, reasonable request.
Synonyms: unsatisfactory, intolerable, inadequate