it means cool, a.k.a. word up
Unlike book (which is often used in texting to mean cool because it comes up when people try to type cool using predictive text), word (and the phrase "word up") also means cool, that's right, or damn straight. It is online jargon, also known as text message shorthand, used in texting, online chat, instant messaging, email, blogs, and newsgroup postings.
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NetLingo Classification: Acronyms and Text Message