(1) Ridin' dirty refers to the act of riding on Las Vegas public transportation (namely the DeuceBus) for an extended period of time without travelling a great distance.
(2) The act of partially soiling one's underpants and continuing to "ride dirty" as though nothing had happened.
Person 1: We've been on this bus for like 20 minutes and we've only gone a block.
Person 2: Ya, we're ridin' dirty.
OR
Person 1: "I think I might be ridin' dirty."
Person 2: "Seriously"?
Person 1: "I dunno, I might be."
for a girl: the act of having great sweaty sex with major ejaculation inside of you, and not taking a shower and proceeding with normal daily tasks (ie: going to work, market, school (college of course), or visiting a friend or OBGYN (OH NO!!).
Female says to friend as they shopping at the mall: omg! I saw Kevin today, and I totally forgot... I'm ridin' dirty!!
Driving while thinking of explicite sex acts to be performed on the personnext to you or someone you might see!; driving with a dirty mind; walking around with a dirty mind.
I can't wait get him home and unzip those pants! I am so Ridin' Dirty!