1.) Similar to the Christian Antichrist, only this is a female who makes it impossible for you to get aroused.
2.) A very sexually un appealing, awkward female who lacks intercourse skills.
When someone sees something that it turns the person off to an extent that it is impossible to get a boner in the next half hour and they are softer than usual.
When the man took the woman to her house he got an antiboner once he discovered her strange obsession with rats that had them roaming around her apartment and inside her bed.