Gargalesis is the scientific term for hard, not-fun tickling. The term (along with 'knismesis', the gentle, more fun kind of tickling) was coined by psychologists Stanley Hall and Arthur Allin in 1897.
Gargalesis involves a mixture of pleasure and pain, and produces responses ranging from recoiling (while smiling, often), to squeals of pain (in human infants). The basic idea applies to many different kinds of animals, including chimpanzees.
As with knismesis, it is impossible to induce gargalesis in yourself unless you are schizophrenic. (See the knismesis node for a bit more on that.)
A woman named Christine R. Harris is the most-quoted tickle researcher; she has even designed a tickling machine. Harris believes that gargalesis plays a role in play-fighting and child development:
"...the smiling and laughter encourage the tickler to continue. If tickling produced a negative facial expression, conspecifics would be far less likely to engage in it during playful bouts -- thus cutting off the development of combat skills that might have survival value."
That is to say, gargalesis is part of the control mechanism for the roughhousing needed to develop combat skills.
Sources: New Scientist, 7 December 2002
Harris, Christine R.; "The Mystery of Ticklish Laughter," American Scientist, 87:344, 1999