Geneva Smitherman, Black Talk: Words and Phrases from the Hood to the Amen Corner (1994) has a series of word-based entries in her glossary of "Black Talk," all signifying much the same thing [combined snippets]:
WORD!/WORD UP! A response of affirmation. Also Word to the Mother! Word Up is the title of a music magazine published in [Paramus,] New Jersey [and incorporated in 1987]. See also WORD IS BORN!
WORD IS BORN! An affirmative response to a statement or action. Also Word!, Word up!, Word to the Mother! A resurfacing of an old familiar saying in the Black Oral Tradition, "Yo word is yo bond," which was popularized by the FIVE PERCENT NATION ["A group established by former members of the Nation of Islam ... in 1964 under the leadership of Clarence 'Pudding' 13X"] in its early years. Word is born! reaffirms strong belief in the power of the word, and thus the value of verbal commitment. One's word is the guarantee, the warranty, the bond, that whatever was promised will actually occur. Born is a result of the AAE pronunciation of "bond" [cross reference omitted].
WORD TO THE MOTHER! A response of affirmation. See also WORD IS BORN!, WORD!/WORD UP!
The Five Percent Nation may have popularized "Yo word is yo bond" at some point after 1964, but the same idea appears in many nineteenth-century texts. The earliest instance in a Google Books search appears in Wesleyan-Methodist Magazine, volume 34 (1811) [combined snippets]:
The Honest Man
He looks not to what he might do, but what he should do. Justice is his first guide ; to which he makes expediency always subservient, altho' the latter is the second law of his actions. He would rather complain than offend ; and hates sin more for the deformity of it than its danger. His simple uprightness works in him that confidence, which often wrongs him, and gives advantage to the subtle and designing ; when he grieves more at their faithlessness than at his own credulity. He hath but one heart, and that lies open to sight ; and were it not discreet, he would not even avoid a witness of his thoughts. His word is his bond, and his yea his oath, which he will not violate through fear or to avoid loss. The untowardness of events may cause him to blame his want of prudence, but can never cause him to eat his promise : neither saith he, "This I saw not," but rather, " This I said."
The original character study "Of the Honest Man," written by Joseph Hall, Bishop of Exeter, appears in Characters of Virtue and Vice (1608); in the original version, Hall uses the phrase "his word is his parchment," not "his word is his bond."
Beginning in the 1840s the equation of one's word with one's bond appears quite frequently. For example, in Jane Smith, Admonitory Epistles from a Governess to Her Late Pupils (1842):
Never prevaricate when you are in the wrong, but quietly submit to correction, dreading most the vengeance of God, if you deny the fault. Never forge a slander to disturb your neighbour's peace. Think before you speak, then you will never have the mortification of wishing your words unsaid. Let it be said of you, "Your word is your bond." When you properly reflect that for every idle word, our Saviour says we must give account, it will make you keep a strict guard over your tongue, and never allow it to speak at random.
Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995) at least corroborates the idea that "Word" originated as an expression of affirmation in U.S. Black slang, though with an interesting twist:
word 1 interj 1980s black teenagers An exclamation of agreement an appreciation, used when someone has said something important or profound: If it's really meaningful, "Word, man, word" should be used—New York Times 2 interj =WORD UP
*word up interj 1980s black An exhortation to listen, to pay attention: Word up, fool. We be fresh tonight—Carsten Stroud [probably based on listen up]
In fuller context, the New York Times quotation above, from an article by Rosemary Breslin titled "City Teen-Agers Talking Up a 'Say What? Storm" (August 29, 1983), reads this way:
Just as popular [as "homeboys"] is "word," which should be said slowly and used only when the person speaking says something with deep meaning. If it's really meaningful, "Word, man, word" should be used.
So whereas Smitherman views "Word" as a single-word affirmation truncated from "Word is bond [or born]" which in turn derived from the older expression "Your word is your bond," Chapman & Kipfer see a fundamental split between "Word" in the sense of "You have said something profound and true" and "Word" in the sense of "Pay attention to the words I am saying."
Not to speak ill of Bill and Ted, whose excellent adventure evidently merited a 90-minute feature-film depiction, but when "Word" enters Valley Dude Speak as channeled by mainstream Hollywood screenwriters, you know it's over as a respectable term of discourse in Black America.