Like Faggot only less severe and more silly or sarcastic.
A very common typo, often since the 's' key is so close to the 'a' key on keyboard.
"Oi! Fasgat! Gimmie a beer, will ya?"
"I threw my back out humping your mom's friend lastnight man, it would have been awesome if your fasgat brother hadn't told me to backoff though"