early 14c. (intransitive) "to be thrilled or tingling," of uncertain origin, possibly a frequentative form of tick (v.) in its older sense of "to touch." The Old English form was tinclian. Some suggest a metathesis of kittle (Middle English kytyllen), from Dutch kietelen, from a common North Sea Germanic word for "to tickle" (compare Old Norse kitla, Old High German kizzilon, German kitzeln).
Meaning "to excite agreeably" (late 14c.) is a translation of Latin titillare. Meaning "to touch lightly so as to cause a peculiar and uneasy sensation in the nerves" is recorded from late 14c.; that of "to poke or touch so as to excite laughter" is from early 15c.; figurative sense of "to excite, amuse" is attested from 1680s. Related: Tickled; tickling. The noun is recorded from 1801. To tickle (one's) fancy is from 1640s.