A combination of 'slut' and 'fantastic'. Describes a person displaying copious amounts of said trait; in this case the individual would be fantastically slutty. User-created words such as this are often casually thrown out at the discretion of the speaker (ex. Fergalicious).
"TJ is slutastic."
"Whore is more fitting, really."
Describes anything or anyone with 'slut-like' qualities.
"Who cares if she's hot, as long as she's slutastic!"
"I had a slutastic time in Vegas"