Breasts. Particularly, very nice ones.
J: Man, Kate is such a bitch.
D: I know, but she has award winning jumpies.
J: Yeah, you know... you're right. I never thought about it like that.
An exercise mainly done by rowers, to condition the legs (quads, specifically). Generally conducted in sets, in the course of the dreaded *circuit*.
Coxwain: 121...122...123... Oh, oops. Sorry guys, I lost count.
Cox: Now all together, we're going to do 200 jumpies, by sets of 50: 1...2...3...
Rowers: We're gonna flip the boat on you, coxwain... shit...