"Ipse is compounded of is and -pse (a pronominal particle of uncertain origin; cf. 145. a), meaning self. The former part was originally declined, as in reapse (for re eapse), in fact. An old form ipsus occurs, with superlative ipsissimus, own self, used for comic effect.”
I cracked up immediately upon reading “ipsissimus,” before even getting to “used for comic effect.” Touche, Romans.