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early 14c. (intransitive) "to be thrilled or tingling," of uncertain origin, possibly a frequentative form of tick (2) 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" (cf. 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" 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.