Its a sexual term to describe a woman when she is in an excited sexual state. Her clit and labia become erect. Fergie's song parallels this to a the two sides of a draw bridge coming down.
The taint. Think of it as the bridge between anustown and vaginaville. obviously, the fergie song is referencing a part of the human anatomy, and the taint is really the only body part that even vaguely resembles a bridge.
A threesome with one guy and two girls. One girl on his face, the other facing her riding him. To complete the move they must be playing with each other's tits.
Dude, last night Jenna and Bailey asked me to join them for a London bridge, it was amazing.
The new location of the London Bridge at Lake Havasu Arizona is home to much promiscuous behavior by young girls and the term "London Bridge" has come to refer to any manner of sexual acts performed by drunk chicks.