Gargalesis is forceful tickling.
Learned men have been arguing about the function of tickling for at least the last couple of millennia and we are no nearer to finding out why it is that when people are tickled they laugh. It can’t be that they enjoy it, because they usually don’t.
In 1999, Christine Harris, a research scientist at the University of California, San Diego, proved that people laugh even when they think they are being tickled by a machine (so it has nothing to do with a person doing it) and that the quality of the laugh was not the same as that provoked by humour. The response is also different when a person is tickled lightly, say by a feather on the sole of the foot or a bug crawling across you. For example, you can’t give yourself a heavy tickle, but you can a light one.
Because the responses are of different kinds, psychologists have contrasted gargalesis, heavy tickling (which is from Greek gargalismos, tickling) with knismesis, the light tickle of a feather. Though both terms date back to an academic article of 1897, they’re extremely rare, and you won’t find either in even the largest dictionary.