Any bizarre, outlandish fetish. Usually a periox is an act of sodomy such as bestiality.

For instance:
Shemale fetishes
Foot fetishes
Sock fetishes
guy-analing-horse fetishes

are all considered to be perioxes whereas

granny fetishes
black guy & asian girl fetishes

are not perioxes.

(noun) A successful type of theft which results in an acceptable, impressive and rewarding payday for the protagonist.