Sadomasochism. When sexual gratification is recieved by inflicting and/or enduring painful activites, this does not have to be exclusively during intercourse. Whips and chains are often considered S&M paraphenalia, as well as bondage collars, spiked jewelry, etc.
May also stand for slave and master.
See also BDSM
Ginny and James are into the S and M scene.