A crass pickup line, typically directed at girls with the name Bridget (for the "Bridge" effect), but can apply to any girl (or guy if that's your bag). Means "I'd like to spread your buttcheeks apart and stick my male sex organ in your anus."
Man: What's your name, cutie?
Man: Oh yeah? I'd like to bridge your buns right open.
*Man gets a drink in the face and a hard kick in the crotch*
Meaghan's a waffle crapper
, I'd bridge her buns right open.