A person with an incredibly repulsive personality. So much so that anyone who has sex with that person will forever be tainted e.g. friends will lose respect for that person, and potential or current partners will no longer be attracted to that person, once they find out that they have had sex with a Lumberg.
In the 1999 movie Office Space, Peter (Ron Livingston) believes that his girlfriend Joanna (Jennifer Anniston) has had sex with his boss, Bill Lumbergh (Gary Cole). Once this information becomes known, he no longer wants anything to do with Joanna and is tormented by the thought of his girlfriend having sex with Lumbergh.
Friend A: "That girl is such a Lumbergh!"
Friend B: "Yeah, this guy I know had sex with her. I can't even look him in the eye anymore."