A sexual partner outside of your primary relationship in which the partner "supplements" what your main relationship is lacking in. The term is used as a less crass version of "fuck buddies" or "friends with benefits."
"My husbandwon't go down on me, but my supplementation partner sure does."
Pajama casual is a term used when working from home and preparing for a video conference. All business above the waist and your pajama pants down below.