A pre-modifier and adjective. Indeed, superlative is a type of adjective but is here modified itself to create a more educated sounding word, and for absolutley no other purpose whatsoever. As such, it describes another adjective as being the greatest of its kind, ie sexy becomes sexiest, and fat becomes fattest. Fattest and sexiest are in themselves regular superlatives requiring no pre-modification, however, superlatively describes a descriptive adjective like sexy or fat to mean that the object being described is the most extreme of its field, be that field sexyness or fatness.
"I love you honey, you're so superlatively sexy"
"You superlatively cocky bastard!"