"tickle" (help. info) evolved from the Middle English tikelen, perhaps frequentative of ticken, to touch lightly. In 1897, psychologists G. Stanley Hall and Arthur Allin described a "tickle" as two different types of phenomena. One type is caused by very light movement across the skin.

What is tickling caused by?

Scientists found being tickled stimulates your hypothalamus, the area of the brain in charge of your emotional reactions, and your fight or flight and pain responses. When you're tickled, you may be laughing not because you're having fun, but because you're having an autonomic emotional response.

Why do humans get ticklish?

Tickling likely serves as a warning signal and training to protect ourselves. It has a secondary feature in humans, other primates, and rats it seems, to facilitate social bonding. But be careful who you tickle, not all animals experience the same enjoyment (some humans don't like it either).