(1) It can be a noun!
(2) It can be a verb!
(3) It can be an adjective!
(4) It can be an adverb!
Pretty much, it can be used in any context at all. It's as openly used as fuck.
2. And then I was all like *sexin* and ran down the street.
3. Look at that tall, sexin, awesome guy walking down the street.
4. John walked sexinly down the street, machine guns ready to go.