Slang term for Cocaine. First used in the movie Scarface, although the word used in the movie is spelled llallo, but is pronounced the same way. Popularized by the 1998 song "Hope I Don't Go Back" from the album "The Element of Surprise" by E-40. This song got a lot of radio play in 1998 and 1999 and popularized this slang term for Cocaine that had previously been used by only a few. The word has also spawned other forms; such as yay, which also means Cocaine and is used in the 1996 song "The Gangsta, The Killa and the Dope Dealer" by "Westside Connection" on the album "Bow Down".
From E-40's song, "I hope I don't go back to slangin' yayo, slangin' yayo, to get my mail." (sounds like mayo not mail, but they are saying mail) meaning money.
Mack 10 from "The Gangsta, The Killa and the Dope Dealer."; "Now what can I say every bitch I lay be pure and Bombay like Peruvian yay, so I brag and I boast man, I got the most man, I make more deleveries than the postman."