c. 1300 (implied in tickling) "to touch lightly so as to cause a peculiar and uneasy or thrilling sensation in the nerves," of uncertain origin, possibly a frequentative form of tick (v.) in its older sense of "to touch." Some suggest a metathesis of Middle English kittle, which is from a shared Germanic word for "to tickle," but tickle is attested earlier. The Old English form was tinclian.
Meaning "to excite agreeably" (late 14c.) is a translation of Latin titillare. Meaning "to poke or touch so as to excite laughter" is from early 15c.; figurative sense of "to excite, amuse" is attested from 1680s. The noun is recorded from 1801. To tickle (one's) fancy is from 1640s. Related: Tickler.