1. Being captivated by someone's buttocks or having one's attention grabbed by looking at someone's bottom. One can be fassinated or find someone's ass fassinating or just caught in fassination.
2. Fantasizing about someone's ass.
When Zeta saw Zatty's bum, he became totally fassinated. The fassination with her bottom persisted all daylong. He would often tell her that he found her fassinating, but she didn't really know what he had in mind.
(adjective) Identifies the subject as noteworthy in some respect, but only for a select group of individuals; to others the subject is likely not even worth knowing, let alone being of any import.
The way Kris Allen sang that song was simply fanscinating!