early 14c., "to comb (flax or hemp) with a heckle;" from heckle (n.) or from related Middle Dutch hekelen. Figurative meaning "to question severely in a bid to uncover weakness" is from late 18c. "Long applied in Scotland to the public questioning of parliamentary candidates" [OED]. Presumably from a metaphor of rough treatment, but also compare hatchel "to harass" (1800), which may be a variant of hazel, the name of the plant that furnished switches for whippings. Related: Heckled; heckling.
"flax comb," c. 1300, hechel, perhaps from an unrecorded Old English *hecel or a cognate Germanic word, from Proto-Germanic *hakila- (source also of Middle High German hechel, Middle Dutch hekel), from PIE root *keg- "hook, tooth."