Green’s Dictionary of Slang

word up! excl.

also say word!

(orig. US black) an excl. of approval, admiration, agreement etc.

[US]Cameo ‘Word Up’ [lyrics] Word up everybody says / When you hear the call you’ve got to get it underway. / Word up it’s the code word. / No matter where you say it you know that you’ll be heard.
[US](con. 1985–90) P. Bourjois In Search of Respect 37: They should just put animals out there patrolling the streets. Word up!
[US]Ebonics Primer at www.dolemite.com [Internet] say word Definition: to concur or agree ie: I second that, short for word up. Example: Yo mike need to stop tripping, say word.
[UK]Guardian Guide 1–6 Jan. 18: Word up, blood.
[UK]Observer Mag. 1 May 43: [pic. caption] Word up: the British Library in London.