Originally originating as latino slang in the 80's to mean "I swear to you on my life what I tell you is true." It eventually was picked up by rappers in the early 90s to mean "I promise." Eventually the all rappers were using it but it wasn't the "in" slang until E-40 used it every other word (in his everyday speach). Eventually it was hear on the movie "Big Money Hustlas" starring ICP and Twiztid where Big Stank (Jamie Madrox) and Lil' Poot (Monoxide) would spout it as there catch phrase. After seeing this movie John Cena decided to use it as his own catch phrase.
Big Stank: It says in Issaiah an shit that they put that dude up on the cross
Lil: poot: WORD LIFE!! SON!!
Big stank: and he was wait till I get down from this mother fucker I'm gonna FUCK me somebody up!
Lil' Poot: It's all skrait
Big Stank: Word Life son!!