ass condom
1) The heterosexual act that helps a friend not get an STD with a woman; Man #1 sticks his male reproductive organ into a Man #2's behind, while that Man #2 puts his penis into the woman, therefore protecting Man #1 from any vaginal diseases.  It's only 97% effective.
2) The Blocking of another cock (less frequently used)
1)- "I don't want to get her fat genes if I have sexual relations with her."
- "It's okay, I'll be your ass condom."
- "Thanks, your the best friend ever."