What is Horse's Arse?
1.
Extremely derogatory term, used to describe a face that is remarkable in it's ugliness.
"Hang on, what's old Horse's arse doing on TV? What, she's marrying Prince Charles?"
2.
Something ugly, used or comparisons.
Her face was the horse's arse.
See
3.
Not a very nice person. Also horse's ass. Can be described as the north end of a horse going south.
I wouldn't take any notice of what he said, he's just a horse's arse.