OTK

From Kinktionary @ Social Kink

OTK stands for Over The Knee, and refers to over the knee spanking which is a common practice in BDSM scenes. Some couples use this in age play scenes to mimic the spankings one receives when they are young. Spanking is used as punishment as well as a source of pleasure for those who like causing or receiving pain.

Many vanilla couples also use spanking as a way to be "naughty" or "kinky" and might not venture into the more complex activities those in the BDSM and fetish partake in.