Dictionary treatments of 'word' and 'word up,' 1994–2005
Geneva Smitherman, Black Talk: Words and Phrases from the Hood to the Amen Corner (1994) has two relevant entries, for word/word up and word is born:
WORD!/WORD UP! A response of affirmation. Also Word to the Mother! Word Up is also the title of a music magazine published in New Jersey. [Cross reference omitted.]
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 [formed in 1964] 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 A[frican] A[mrican] E[nglish] pronunciation of "bond"; [cross reference omitted].
Smitherman also has this entry for G:
G 1) A form of address for a male, usually one who is HIP or DOWN. Probably the AAE version of "guy." Also man (older term); money (newer term). 2) A woman a man has a relationship with.
I should note that Smitherman offers this reading of G while fully aware of the term OG, which has this entry in her book:
OG Original Gangster; a gang member who has earned PROPS because of his bold actions.
Clarence Major, Juba to Jive: A Dictionary of African American Slang (1994) has these entries:
Word interj. (1950s–1990s) affirmation spoken in agreement; the truth; street culture gospel. (T[erry] W[illiams], [The] C[ocaine] K[ids: The Inside Story of a Teenage Drug Ring (1989)], p. 138.) Example: "Word! I was there, I aw it with my own eyes." S[outhern] C[ity] U[se], P[imp and] P[rostitute] U[se], Y[outh] C[ulture] U[se], D[rug] C[ulture] U[se].
Word up! interj. (1980s–1990s) call for attention; used as an exression of one's word of honor. (W[illiam] K[.] B[entley and] J[ames] M[.] C[orbett], P[rison] S[lang: Words and Expressions Depicting Life Behind Bars (1992)], p.51.) Example: "Word up, the cops are down there right now busting Rickie." S[outhern] C[ity] U[se].
Major does not have an entry for G in the sense of "guy," although he does include entries for G as a noun meaning "a thousand dollars" (from the 1940s–1950s) and as a verb meaning "to have sexual intercourse with" (1990s). Major's entry for O.G. notes only its meaning (from the period 1900s–1950s) "Old Girl; mother." Smitherman dates "yo word is yo bond" to 1964, and Major dates "word" (as affirmation) to the 1950s. Nevertheless, Major's earlier Dictionary of Afro-American Slang (1970) has no entry for any form of word.
It's interesting that, publishing in the same year (1994), Professor Smitherman of Michigan State sees "Word" and "Word up" (and "Word is born") as essentially interchangeable expressions of agreement, while Professor Major of the University of California at Davis, sees "Word" as an expression of agreement but "Word up" as, in the first instance, a call for attention, and, in the second, an attestation along the lines of "I swear."
This inconstancy underscores an essential problem with defining slang words: Since they don't show up in popular use neatly predefined, they are subject to multiple interpretations by the people who hear and adopt them; as a result, it is not at all unlikely that a term may mean one thing in one locale and another in another. This phenomenon might serve as a caution to authors not to assume that a slang term's usage across a diverse but definable group (such as "African Americans") is settled and uniform—especially in its early years of propagation.
Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995) offers these relevant entries:
word 1 interj 1980s black teenagers An exclamation of agreement and appreciation, used when someone has said something important or profound: If it's really meaningful, "Word, man, word" should be used—["City Teen-Agers Talking Up a 'Say What?' Storm"] New York Times [(August 29, 1983)] 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 [Close Pursuit (1987)] {probably based on listen up}
Chapman & Kipfer cites instances from the 1980s that corroborate Major's claim that "word" and "word up" had different senses in at least some parts of the United States. Nevertheless, ten years later, Jeremy Sideris & Brittany McWilliams, From Grill to Dome: A Dictionary of African American Slang Words and Phrases (2005) indicates that the Smitherman view (that the terms "word" and "word up" have essentially the same meaning) has prevailed in the broader marketplace of African American English speech:
Word: Statement of agreement. See also booyah, down, fo' shizzle my nizzle, fo' zizzle my nizzle, really though, true dat, and word-up.
Word-up: Strong statement of agreement. See also booyah, down, fo' shizzle my nizzle, fo' zizzle my nizzle, really though, true dat, and word.
According to Sideris & McWilliams, the only difference between the two words is in the degree of strength implied in the statement of agreement.
A note on 'word is bond'
With regard to Smitherman's comment that "yo word is yo bond" is "an old familiar saying in the Black Oral Tradition," that familiarity may be due to the fact that the same essential idea has been a proverb in English since at least 1500. From Fred Shapiro, The Yale Book of Quotations (2006):
A man's word is his bond. Lancelot of Lake (ca. 1500). This source, with the wording "o kingis word shuld be o kingis bonde," is the earliest version given by the Oxford Dictionary of Proverbs.
The first several citations from the full entry for this proverb in J.A. Simpson, The Concise Oxford Dictionary of Proverbs (1982) deserve mention:
An Englishman's word is his bond c 1500 Lancelot of Lake (STS) l. 1671 O kingis word shuld be o kingis bonde. a 1606 Nobody and Somebody C2v Nobodis worde is as good as his bond. 1642 T[homas] Fuller Holy State v. xiii. He hath this property of an honest man, that his word is as good as his band. 1754 [Samuel] Richardson [The History of Sir Charles] Grandison I. Letter ix. I am no flincher ... The word of Sir Rowland Meredith is as good as his bond. 1841 [Charles] Dickens [The] Old Curiosity Shop lviii. 'Marchioness,' said Mr. Shriveller [Swiveller, actually], rising, 'the word of a gentleman is as good as his bond—sometimes better, as in the present case.' 1924 G. Benham Book of Quotations (rev. ed.) 735 (Proverb) An Englishman's word is his bond.
I have no doubt that, as Smitherman asserts, an adage along the lines of "Your word is your bond" was common among black families in the United States long before "word" appeared as (arguably) a truncated form of the same idea. But it also seems highly likely that the wording entered the pool of African American English folk wisdom from a proverb that originated in England.