A phrase made popular by Vanilla Ice
in the early 90's
The story I was told, that I believe is true, is that this phrase originated as "Word To Your Motherland", something for people of different cultures to say to each other. "Word" in itself is sort of affirmative identifier, something you would say to show approval. At least that's what it's become over the years. Originally, I'm sure it meant something like "Send word of me".. sort of like "Regards to your mother".
At any rate, Vanilla Ice bastardized it to its shortened form. He's more-than-likely not the originator of the phrase, but he is more-than-likely the guy that got every 7th grader in 1990 saying it. There was even a Saturday Night Live skit where Kevin Bacon, dressed up as Vanilla Ice, went on Nat X and said "Word to your mother" after everything.
And to top it all off, the rap band 3rd Bass wrote a song called "Pop Goes The Weasel" about how Vanilla Ice is a rip-off artist (see "Under Pressure") and started their video off with a spoof of Mr. Van Winkle himself saying
"You know I'm legit, you know why? 'Cause I'm from the streets... Word to your mother's uncle"
Word to your mother.