Mermaid play
From wipipedia.org
Mermaid play is a bondage scene where one or more people (usually female) pretend to be mermaids.
Mermaids are usually depicted as naked females from the waist up and having fish tails below the waist. Thus in mermaid play, the victim is naked and has her legs firmly bound together so as to make a tail. Splints or other devices may be used so that she cannot bend her knees, making her even more helpless.
