When someone loses all common sense in regards to something, because they have been in a constraining situation keeping them from doing it for so long. Usually refers to when someone has been in a relationship for so long they forget how to get with members of the opposite sex.
John: Ever since Joe and his ex broke up, he's hasn't been able to get any poon.
Josh: What can you expect? He was with her for so long that he's become freetarded.