…come to be slang for “well said,” and/or “I agree?”
Early 80’s.
Cameo is partly responsible for it’s proliferation.
I always thought it was shorthand for “I agree with what you say”, or “I agree with your word(s)”, or “Your words are true”.
It’s just much hipper to shorten it to “Word”.
Word up!
I personally first saw it in one of the “Lethal Weapon” movies (not the first).
I always assumed it was a shortening of the phrase, “Word to your mother.”
According to the Oxford English Dictionary, the phrase has multiple origins with similar meanings. 1st attributed to a Scottish saying “O kingis word shuld be o kingis bonde”
And to a common phrase uttered by prisoners in the 20th century: “My word is my bond”
And more recently the slang origin is attributed to Big Daddy Kane “Word to the mother (land)” and Dr. Dre"Word to the motherf*cker straight outta Compton" in 1988 and as mentioned earlier: “Word up!” by Cameo and a 1985 article in NY Magazine
Word/word up/word to your mother: expressing affirmation, agreement, or admiration
i am pretty close to positive that word is the product of shortening the phrase “word is bond” popularized by the nation of the 5%. the nation of the 5% was a religious movement with origins in Harlem. Their doctrine and rhetoric began influencing hip hop acts by the early 80’s
Hah! I just heard this on “Do You Speak American?” this evening. 
I also had no idea that Clueless was responsible, for, like, so much teenagespeak.
//werd.
We began using “Word is bond” in 6th grade (1982) every time someone said anything remotely close to being true. We had no idea of its origins. It was just something to say.