1.  A physician or surgeon; a professor of the art of healing. Spenser. [1913 Webster]
"Leech, heal thyself."

2.  Any one of numerous genera and species of annulose worms, belonging to the order Hirudinea, or Bdelloidea, esp. those species <-- formerly! --> used in medicine, as Hirudo medicinalis of Europe, and allied species. [1913 Webster]
" In the mouth of bloodsucking leeches are three convergent, serrated jaws, moved by strong muscles. By the motion of these jaws a stellate incision is made in the skin, through which the leech sucks blood till it is gorged, and then drops off. The stomach has large pouches on each side to hold the blood. The common large bloodsucking leech of America (Macrobdella decora) is dark olive above, and red below, with black spots. Many kinds of leeches are parasitic on fishes; others feed upon worms and mollusks, and have no jaws for drawing blood. See Bdelloidea. Hirudinea, and Clepsine." [1913 Webster]

3.  A glass tube of peculiar construction, adapted for drawing blood from a scarified part by means of a vacuum. [1913 Webster]

1.  To treat as a surgeon; to doctor; as, to leech wounds. [1913 Webster]

2.  To bleed by the use of leeches. [1913 Webster]

leech, n.
1 any freshwater or terrestrial annelid worm of the class Hirudinea with suckers at both ends, esp. Hirudo medicinalis, a bloodsucking parasite of vertebrates formerly much used medicinally.
2 a person who extorts profit from or sponges on others.

Idiom:
like a leech persistently or clingingly present.

leech, n. archaic or joc. a physician; a healer.

leech, n.
1 a perpendicular or sloping side of a square sail.
2 the side of a fore-and-aft sail away from the mast or stay.

Etymology:
ME, perh. rel. to ON lik, a nautical term of uncert. meaning

N  remedy, help, cure, redress, medicine, medicament, diagnosis, medical examination, medical treatment, surgery, preventive medicine, clinical thermometer, stethoscope, X-ray machine, anthelmintic, antidote, antifebrile, antipoison, counterpoison, antitoxin, antispasmodic, bracer, faith cure, placebo, helminthagogue, lithagogue, pick-meup, stimulant, tonic, vermifuge, prophylactic, corrective, restorative, sedative, palliative, febrifuge, alterant, alterative, specific, antiseptic, emetic, analgesic, pain-killer, antitussive, antiinflammatory, antibiotic, antiviral, antifungal, carminative, Nepenthe, Mithridate, cure, treatment, regimen, radical cure, perfect cure, certain cure, sovereign remedy, examination, diagnosis, diagnostics, analysis, urinalysis, biopsy, radiology, medicine, physic, Galenicals, simples, drug, pharmaceutical, prescription, potion, draught, dose, pill, bolus, injection, infusion, drip, suppository, electuary, linctus, lincture, medicament, pharmacon, nostrum, receipt, recipe, prescription, catholicon, panacea, elixir, elixir vitae, philosopher's stone, balm, balsam, cordial, theriac, ptisan, agueweed, arnica, benzoin, bitartrate of potash, boneset, calomel, catnip, cinchona, cream of tartar, Epsom salts, feverroot, feverwort, friar's balsam, Indian sage, ipecac, ipecacuanha, jonquil, mercurous chloride, Peruvian bark, quinine, quinquina, sassafras, yarrow, salve, ointment, cerate, oil, lenitive, lotion, cosmetic, plaster, epithem, embrocation, liniment, cataplasm, sinapism, arquebusade, traumatic, vulnerary, pepastic, poultice, collyrium, depilatory, emplastrum, eyewater, vesicant, vesicatory, compress, pledget, bandage, treatment, medical treatment, regimen, dietary, dietetics, vis medicatrix, vis medicatrix naturae, medecine expectante, bloodletting, bleeding, venesection, phlebotomy, cupping, sanguisae, leeches, operation, surgical operation, transfusion, infusion, intravenous infusion, catheter, feeding tube, prevention, preventative medicine, immunization, inoculation, vaccination, vaccine, shot, booster, gamma globulin, pharmacy, pharmacology, pharmaceutics, pharmacopoeia, formulary, acology, Materia Medica, therapeutics, posology, homeopathy, allopathy, heteropathy, osteopathy, hydropathy, cold water cure, dietetics, surgery, chirurgery, chirurgy, healing art, leechcraft, orthopedics, orthopedy, orthopraxy, pediatrics, dentistry, midwifery, obstetrics, gynecology, tocology, sarcology, hospital, infirmary, pesthouse, lazarhouse, lazaretto, lock hospital, maison de sante, ambulance, dispensary, dispensatory, drug store, pharmacy, apothecary, druggist, chemist, Hotel des Invalides, sanatorium, spa, pump room, well, hospice, Red Cross, doctor, physician, surgeon, medical practitioner, general practitioner, specialist, medical attendant, apothecary, druggist, leech, osteopath, osteopathist, optometrist, ophthalmologist, internist, oncologist, gastroenterologist, epidemiologist, public health specialist, dermatologist, podiatrist, witch doctor, shaman, faith healer, quack, exorcist, Aesculapius, Hippocrates, Galen, accoucheur, accoucheuse, midwife, oculist, aurist, operator, nurse, registered nurse, practical nurse, monthly nurse, sister, nurse's aide, candystriper, dresser, bonesetter, pharmaceutist, pharmacist, druggist, chemist, pharmacopolist, remedial, restorative, corrective, palliative, healing, sanatory, sanative, prophylactic, preventative, immunizing, salutiferous, medical, medicinal, therapeutic, chirurgical, epulotic, paregoric, tonic, corroborant, analeptic, balsamic, anodyne, hypnotic, neurotic, narcotic, sedative, lenitive, demulcent, emollient, depuratory, detersive, detergent, abstersive, disinfectant, febrifugal, alterative, traumatic, vulnerary, allopathic, heteropathic, homeopathic, hydropathic, anthelmintic, antifebrile, antiluetic, aperient, chalybeate, deobstruent, depurative, laxative, roborant, dietetic, alimentary, nutritious, nutritive, peptic, alexipharmic, alexiteric, remediable, curable, aux grands maux les grands remedes, Dios que da la llaga da la medicina, para todo hay remedio sino para la muerte, temporis ars medicina fere est, the remedy is worse than the disease, throw physic to the dogs, I'll none of it.

N  advice, counsel, adhortation, word to the wise, suggestion, submonition, recommendation, advocacy, advisement, exhortation, expostulation, admonition, guidance, instruction, charge, injunction, obtestation, Governor's message, President's message, King's message, Queen's speech, message, speech from the throne, adviser, prompter, counsel, counselor, monitor, mentor, Nestor, magnus Apollo, senator, teacher, guide, manual, chart, physician, doctor, leech, archiater, arbiter, reference, referment, consultation, conference, pourparler, recommendatory, hortative, dehortatory, admonitory, Int, go to!, give every man thine ear but few thy voice, I pray thee cease thy counsel, my guide, philosopher, and friend, 'twas good advice and meant, my son be good, verbum sat sapienti, a word to the wise is sufficient, vive memor leti, we, ask advice but we mean approbation.

leech onto, medicinal leech