The phrase "unforgivable act" refers to something terrible or wrong that a person does, which is so bad that others find it impossible to forgive them for it. Full definition
The day before, I had called for the elimination of assigned schools in favor of universal choice — a nearly unforgivable act as far as many suburban parents are concerned. (educationnext.org)
And soon, cracks were appearing in their complex relationship... until one unforgivable act leads to another and a partnership begins to unravel. (bookbrowse.com)
SEOUL (Reuters)- North Korea said on Saturday U.S. missile strikes against a Syrian airfield on Friday were «an unforgivable act of aggression» that showed its decision to develop nuclear weapons was «the right choice a million times over.» (businessinsider.com)