early 14c. (intrans.) "to be thrilled or tingling," of uncertain origin, possibly a frequentative form of tick (2) in its older sense of "to touch." The OE form was tinclian. Some suggest a metathesis of kittle (M.E. kytyllen), from Du. kietelen, from a common North Sea Gmc.
word for "to tickle" (cf. O.N. kitla, O.H.G. kizzilon, Ger. kitzeln). Meaning "to excite agreeably" (late 14c.) is a translation of L. 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. The noun is recorded from 1801. Ticklish in the lit. sense of "easily tickled" is recorded from 1598, later than the fig. sense (1580s); an earlier word for this was tickly (1520s). Tickled "pleased, happy" is from 1580s.