applied to human parasites since 1784.
obsolete for "physician," from O.E. læce, from O.Dan. læke, from P.Gmc. *lælijaz "healer, physician" (cf. O.N. læknir, O.H.G. lahhi, Goth. lekeis "physician"), lit. "one who counsels," perhaps connected with a root found in Celt. (cf. Ir. liaig "charmer, exorcist, physician")
and/or Slavic (cf. Serbo-Croatian lijekar), with an original sense of "speak, talk, whisper, conjurer." The form and sense merged with leech
(1) in M.E. by folk etymology. In 17c., leech usually was applied only to veterinary practitioners. The third finger of the hand, in O.E., was læcfinger, translating L. digitus medicus, Gk. daktylus iatrikos, supposedly because a vein from that finger stretches straight to the heart.