This portmanteau is used to describe a person who is thought to be good looking and would normally be described as 'fit', however, there is someone else in earshot who would take personal offence (boyfriend or parent) and therefore 'nice' is quickly inserted as a substitute.
Ben: Yooo, that girl over there is f'...
*Nathan (girl's boyfriend comes over)*
Ben (continued): f'nice. Hey, s'up Nathan!
ficey is an combination of the 2 words "fruity" (as in gay) + "spicey" (as in hot), this combination was created in a groupchat (I AM NOT IN IT SO IT IS NOT AN INSIDE JOKE) who also has a tiktok account and spreaded this word and now people use it that arent in the groupchat (me).