A man of low standards in reguards to sexual partners. A true Shagnasty will quite happily have sex with someone he finds physically repulsive. A shagnasty never admits to sleeping with ugly girls and to most people may seem completly celabet.
"you hear about john?"
"He got caught at the old peoples home trying to organise a geriactric orgy."
"lol, that guy's a shagnasty"