Thumbation
When couples are holding hands and then start touching or petting each others' thumbs. Can evolve into thumb wars and other kinky behaviors.
Man Friend: "So what did you do at the movies with Katie at the movies last night?"
Dude: "We held bands. Little thumbation."
Man Friend: "Kinky."
Dude: "We held bands. Little thumbation."
Man Friend: "Kinky."