to touch (a person or a body part) lightly and repeatedly so as to excite the surface nerves and cause uneasiness, laughter, or spasmodic movements
to excite (somebody) agreeably
to provoke (somebody) to laughter
to catch (a trout) in one's hands
to have or cause a tingling or prickling sensation
tickle pink/tickle pink/to death
informal to amuse or delight (somebody)
tickler noun
tickly adj
[Middle English tikelen to be delighted or thrilled, prob from tick2 in the obsolete sense ‘to touch lightly’]