Learn English – the etymology of “word!”

Word!

Dig a bit deeper into the Urban Dictionary and you will find this:

word to your mother: An anachronistic corruption of the phrase "word to the mother", which was a popular reference to Africa or "The Motherland" during the late 1980s Afrocentric movement. While the replacement of "the" with "your" effectively obliterated the term's Afrocentric roots, it continued to be used in the same manner, that is, to express agreement. Alternatively, the "your" could take on sinister connotations, implying that speaker was sexually intimate with the listener's mother, as in "say hi to your mom for me", or, in keeping with the whack terminology, "props to your mom, she's da bomb". Finally, the phrase might mean nothing at all, and be used to ineptly feign street cred, in the style of Vanilla Ice.

FYI: Big Daddy Kane has a song "Word to the Mother(Land)," which you can hear here.

You will also find this explanation in Roc the Mic Right: The Language of Hip Hop Culture:

The African American oral tradition is rooted in a belief in the power of the Word. The African concept of Nommo, the Word, is believed to be the force of life itelf. To speak is to make something come into being. Once something is given the force of speech, it is binding—hence the familiar saying "Yo word is yo bond," which in today's Hip Hop Culture has become WORD IS BORN. The Hip Hop expressions WORD, WORD UP, WORD TO THE MOTHER, and similar phrases all stem from the value placed on speech. Creative, highly verbal talkers are valued.

This supports the comment from UD that it came from "my word is my bond," or a reasonable facsimile of the expression, but only in part. In 20th/21st century African American culture, "word" collocates broadly and creatively, and untangling its Genesis is a tricky business.