Sex with someone in your own dorm.
Unlike Nick D, every definition doesn't require bitterness relating to Stanford.
Sex between two (or more) people living in the same dorm.
Common among freshman, who excited by their newfound freedom have sex with the nearest person they can get their booze-soaked hands on. Its all well and good until October rolls around and you hate each other but still see each other everyday.
"You fucked your neighbor? Bad idea, dormcest usually does not end well."
"But we were drunk! And my mom wasnt going to walk in on us!"
hooking up with someone who lives in your dorm or on the same floor as you...usually initiated by drunk decisions.
Did you hear that Kevin is hooking up with this girl two doors down? Thats totally going on our dormcest chart.