Its kind of hard to work out where a phrase like that comes from.
A bond is something you give to give commitment on a promise, thus word is bond means just that, that your word is good enough to count as a bond. Its hard to pinpoint where it came from because it is not slang.
In terms of Hip-Hop use, it becamse used by blacks in the Nation of Islam and Five Percenters, as it is used in their lessons, and then becamse a street thing, much like calling someone "God."
"Have you not learned that your word is bond? Yes, my word is bond and bond is life, I shall give my life, before my word shall fail.