When three men work together in a cooperative sexual act where two men create a common hand-vagina (referred to as the "rig") and allow the third man (referred to as the "Jimmy") to insert his penis into their co-created hand-vagina.
In some cases, the hand vagina can be fucked by one of the two participating hand-vagina creators.
In a third (untested case), the two hand-vagina creators can alternate fucking the hand-vagina in a rhythmic and sexual motion.
In certain environments, such as the Ménage à trois you can encounter the rare and unseen co-ed Jennyrig; the co-ed Jennyrig is when two women co-create the hand-vagina and the man inserts his penis into the women's hands, eventually climaxing (hopefully).
Carl and Jeff made me the best jimmyrig for my birthday!
I haven't had a tighter jimmyrig in 30 years.
I "jimmyrigged" with my buddies, Dave and Matt last night. Luckily, I wasn't part of the rig.
Jerry taught his sons the meaning of sex by "jimmyrigging" them last night.