Shortened version of "Word is bond" which is shortened version of "My word is my bond".
Meaning what you say is true and good.
However it can be used as a equivalent to saying "for real" or "are you serious".
Man 1: Yo man I tapped that ass last night
Man 2: Really?
Man 1: Wordbond
Dude 1: Hey man I almost hooked up with Jessica last night
Dude 2: Wordbond?
Dude 1: Yah man