1. The word "promo" is short for "promotional".
This word is most known in the Urban scene for being seen on vinyl (LPs, records), tapes and CDs.
Back in the early days of Hip-Hop vinyl production (and still to this day), vinyls were/are given out to DJs known as "promo vinyls". These vinyls are cut (the grroves in vinyls are "cut" using a special type of needle) and given out, usually for free to DJs so that a DJ can play the vinyl in a club or on the radio.
On these vinyls, is the word "promo" written on the record sleeve or record label to tell people that this is a "promo" vinyl that this DJ has. It mostly has "for promotional use only. Not for resale" written on the record case or label instead of just the abbriviated "promo".
2-Expression 1. Promo is sometimes known to people as an expression to explain a persons seemingly over-the-top personality.
The word promo is sometimes known as a "professional homo (promo)" in the way that the person seems to be so good at acting homosexual.
Expression 2. It can also be used as an expression of saying that a person likes to promote his/herself too much. This version of the expression comes from the word "promote".
1. Example; "Hey man, check this out, I got a Biggie promo on vinyl today".
2-Expression 1. "That dude is so obviously a promo!"
Expression 2. "That chick always like to promo herself around the place too much..."