Props is short for "propers" as in, "proper respect"
On 13 Dec 2001, Joe M**** answered the question on Deja News. Here is how it is explained:
Props, short for Propers, -Don't forget that the entire word "propers" is used in the song "Respect", written by Otis Redding and most famously recorded by Aretha Franklin in 1967. At least, I think it is ("all I'm askin' in return honey is to give me my propers when I get home")-