"to question severely in a bid to find weaknesses," 1788, transferred usage of hekelen "to comb (flax or hemp) with a heckle," from M.E. hekele "a comb for flax or hemp" (early 14c.), from M.Du. hekelen, the original sense of which was "to prickle, irritate," from P.Gmc. *khakilo- (related to hackle). "Long applied in Scotland to the public questioning of parliamentary candidates" [OED].