In the 60's, a girl would get pinned by a guy when he made her his girlfriend. Ya know, after many dates, and the guy was ready to call her his girlfriend, he would formalize it by putting a silvery pin on her. So getting "pinned" means u are now taken. Watch the Dick Van Dyke version of Bye Bye Birdie, especially the song "The Telephone Hour." They use the term pinned quite a lot.