To have sex with a friend or acquaintance's mother; Usually as part of a single, ill-advised, drunken episode. Sometimes resulting in unplanned pregnancy.
Origin: rumors that James Doohan (actor of Star Trek fame) commonly had sexual intercourse with the mothers of his Star Trek castmates (crew).
Mostly used within the phrase "pulling a Doohan"
Hey LeBron, I heard that Delonte West 'pulled a Doohan' last night.
A person with an incredibly small, and usually bent penis, often a "Doohan" will try to make up for this with fast cars or big muscles.
I thought he was really hot until he took his pants off and I realised he was just a Doohan.