Meaning "Down To Build A Bridge". Building a bridge is the sex act involving 2 guys and a girl. The guys stand at opposite ends of the girl, one giving it to her doggy style, the other receiving a blowjob. The guys then clasp hands, thus completing the bridge. Also, there is an anal variation.
In the same vein as Eiffel towering, pig roasting, golden gating...
"Yo Josh, you think Amelia would be DTBAB?"
"Well Campbell, she was last week."