a) to agree with
b) without a doubt
R: You know your momma's gonna have a fit when she finds that you burned down the house.
R: Your father will hate you once he finds that you had an accident in his Lincoln that he only drives on Sundays.
R: Is the sky blue?
"Word?" ("You speak the truth?")
"Word." ("I speak the truth.")
"Word" has no single meaning, but is used to convey a casual sense of affirmation, acknowledgement, agreement, or to indicate that something has impressed you favorably.
Its usage among young blacks has been parodied ad nauseam among clueless suburban whites.