When someone/something has really annoyed you.
"God this job is a fucking COAT"
This acronym was originally developed in the town of Melbourne, Australia in mid 2006 as a means of describing a strongly unpleasant, offensive, or undesirable female, or a female displaying qualities considered highly undesirable at a given moment.
The beauty of this term is its adaptable nature, and its subjective application.
Also, this term allows one to use a highly offensive term towards someone, in a fashion that does not sound offensive. To this end it is often found in use in offices, professional environments, and there is also one unsubstantiated account of this term being used in a Supreme Court towards the presiding judge.
"I had to get out of the house for a while Brad, the missus is being a COAT..."
"I cant believe you baby; you are really acting like a COAT right now. Pull your head in!"
Her - "Youre such a bastard....just buy me an engagement ring already, it's about time after 4 years together..."
Him - "For fucks sake Candace, you are an absolute COAT...we're over bitch! Pack your shit and hit the bricks."
Or That COAT Sally would stop calling me last night. she needs to get a life!