A sexual act involving 2 men and 1 woman with large, floppy breasts. The woman lays on her back, while each man slides his penis under a breast from each side. To avoid touching each other in the middle, the men coordinate going back and forth, like a see-saw.
May also be referred to as "Lumberjacking"
Man 1: "I'd fuck that fatty *under* her floppy tits."
Man 2: "Me too - let's see-saw her!"