To be loyal to one organization or company for a long time, only to abruptly switch to the opposing organization or company.
Named for PA Senator Arlen Specter who switched from Republican to Democrat after 20 years as a Republican.
Bob: Did you hear about George?
Phil: No, what happened?
Bob: George was caught pulling a specter. He just traded his Impala for a Ford Taurus.
When you abandon your party, and join the oppisition.
Dude I can't believe Jimmy is going to leave the GOP and become a dirty Democrat, he really is going pull a specter.