Etymology
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leech (v.)
"to cure, heal," c. 1200, from Old English also had a verb læcnian, from the source of leech (n.2). Meaning "to apply leeches medicinally" is from 1802 (implied in leeching), from leech (n.1). Related: Leeched.
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leech (n.1)
"bloodsucking aquatic worm," from Old English læce (Kentish lyce), of unknown origin (with a cognate in Middle Dutch lake). Commonly regarded as a transferred use of leech (n.2), but according to OED the Old English forms suggest this is a distinct word, which has been assimilated to leech (n.2) by folk etymology. Figuratively applied to human parasites since 1784.
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leech (n.2)

"physician" (obsolete, poetical, or archaic), from Old English læce "leech," probably from Old Danish læke, from Proto-Germanic *lekjaz "enchanter, one who speaks magic words; healer, physician" (source also of Old Frisian letza, Old Saxon laki, Old Norse læknir, Old High German lahhi, Gothic lekeis "physician"), literally "one who counsels," perhaps connected with a root found in Celtic (compare Irish liaig "charmer, exorcist, physician") and Slavic (compare Serbo-Croatian lijekar, Polish lekarz), from PIE *lep-agi "conjurer," from root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')."

For sense development, compare Old Church Slavonic baliji "doctor," originally "conjurer," related to Serbo-Croatian bajati "enchant, conjure;" Old Church Slavonic vrači, Russian vrač "doctor," related to Serbo-Croatian vrač "sorcerer, fortune-teller." The form merged with leech (n.1) in Middle English, apparently by folk etymology. In early Middle English the word also was used of God and Christ; but by 17c. the sense had so deteriorated that leech typically was applied only to veterinary practitioners, and soon it was entirely archaic.

The fourth finger of the hand, in Old English, was læcfinger, translating Latin digitus medicus, Greek daktylus iatrikos, supposedly because a vein from that finger stretches straight to the heart.

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leechcraft (n.)
"art of healing," Old English læcecræft; see leech (2) + craft (n.). Old English also had læcedom "medicine." A later word for it was leechery (c. 1600).
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doctor (n.)
Origin and meaning of doctor

c. 1300, doctour, "Church father," from Old French doctour and directly from Medieval Latin doctor "religious teacher, adviser, scholar," in classical Latin "teacher," agent noun from docere "to show, teach, cause to know," originally "make to appear right," causative of decere "be seemly, fitting" (from PIE root *dek- "to take, accept").

Meaning "holder of the highest degree in a university, one who has passed all the degrees of a faculty and is thereby empowered to teach the subjects included in it" is from late 14c. Hence "teacher, instructor, learned man; one skilled in a learned profession" (late 14c.).

The sense of "medical professional, person duly licensed to practice medicine" (replacing native leech (n.2)) grew gradually out of this from c. 1400, though this use of the word was not common until late 16c. The transitional stage is exemplified in Chaucer's Doctor of phesike (Latin physica came to be used extensively in Medieval Latin for medicina).

That no man ... practyse in Fisyk ... but he be Bacheler or Doctour of Fisyk, havynge Lettres testimonyalx sufficeantz of on of those degrees of the Universite. [Rolls of Parliament, 1421]

Middle English also used medicin for "a medical doctor" (mid-15c.), from French. Similar usage of the equivalent of doctor is colloquial in most European languages: Italian dottore, French docteur, German doktor, Lithuanian daktaras, though these typically are not the main word in those languages for a medical healer. For similar evolution, compare Sanskrit vaidya- "medical doctor," literally "one versed in science." German Arzt, Dutch arts are from Late Latin archiater, from Greek arkhiatros "chief healer," hence "court physician." French médecin is a back-formation from médicine, replacing Old French miege, from Latin medicus.

Phrase what the doctor ordered "just the thing" is attested by 1914.

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*leg- (1)

Proto-Indo-European root meaning "to collect, gather," with derivatives meaning "to speak" on the notion of "to gather words, to pick out words."

It forms all or part of: alexia; analects; analogous; analogue; analogy; anthology; apologetic; apologue; apology; catalogue; coil; colleague; collect; college; collegial; Decalogue; delegate; dialect; dialogue; diligence; doxology; dyslexia; eclectic; eclogue; elect; election; epilogue; hapax legomenon; homologous; horology; ideologue; idiolect; intelligence; lectern; lectio difficilior; lection; lector; lecture; leech (n.2) "physician;" legacy; legal; legate; legend; legible; legion; legislator; legitimate; lesson; lexicon; ligneous; ligni-; logarithm; logic; logistic; logo-; logogriph; logopoeia; Logos; -logue; -logy; loyal; monologue; neglect; neologism; philology; privilege; prolegomenon; prologue; relegate; sacrilege; select; syllogism; tautology; trilogy.

It is the hypothetical source of/evidence for its existence is provided by: Greek legein "to say, tell, speak, declare; to count," originally, in Homer, "to pick out, select, collect, enumerate;" lexis "speech, diction;" logos "word, speech, thought, account;" Latin legere "to gather, choose, pluck; read," lignum "wood, firewood," literally "that which is gathered," legare "to depute, commission, charge," lex "law" (perhaps "collection of rules"); Albanian mb-ledh "to collect, harvest;" Gothic lisan "to collect, harvest," Lithuanian lesti "to pick, eat picking;" Hittite less-zi "to pick, gather."

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veterinarian (n.)

"animal doctor, one who practices the art of treating disease and injuries in domestic animals," 1640s, from Latin veterinarius "of or having to do with beasts of burden," also, as a noun, "cattle doctor," from veterinum "beast of burden," perhaps from vetus (genitive veteris) "old" (see veteran), possibly from the notion of "experienced," or of "one year old" (hence strong enough to draw burdens). Another theory connects it to Latin vehere "to draw," on notion of "used as a draft animal." Replaced native dog-leech (1520s).

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finger (n.)

"terminal or digital member of the hand" (in a restricted sense not including the thumb), Old English finger, fingor "finger," from Proto-Germanic *fingraz (source also of Old Saxon fingar, Old Frisian finger, Old Norse fingr, Dutch vinger, German Finger, Gothic figgrs "finger"), with no cognates outside Germanic; perhaps ultimately from PIE root *penkwe- "five."

As a unit of measure for liquor and gunshot (late Old English) it represents the breadth of a finger, about three-quarters of an inch. They generally are numbered from the thumb outward, and named index finger, fool's finger, leech- or physic-finger, and ear-finger.

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*dhe(i)- 

*dhē(i)-, Proto-Indo-European root meaning "to suck."

It forms all or part of: affiliate; affiliation; effeminate; effete; epithelium; fawn (n.) "young deer;" fecund; fellatio; Felicia; felicitate; felicity; Felix; female; feminine; femme; fennel; fenugreek; fetal; feticide; fetus; filial; filiation; filicide; filioque; fitz; infelicity.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dhayati "sucks," dhayah "nourishing;" Greek thēlē "mother's breast, nipple," thēlys "female, fruitful;" Latin felare "to suck," femina "woman" ("she who suckles"), felix "happy, auspicious, fruitful," fetus "offspring, pregnancy;" fecundus "fruitful, fertile, productive; rich, abundant;" Old Church Slavonic dojiti "to suckle," dojilica "nurse," deti "child;" Lithuanian dėlė "leech;" Old Prussian dadan "milk;" Gothic daddjan "to suckle;" Old Swedish dia "suckle;" Old High German tila "female breast;" Old Irish denaim "I suck," dinu "lamb."

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echidna (n.)

Australian egg-laying hedgehog-like mammal, 1810, said to have been named by Cuvier, usually explained as from Greek ekhidna "snake, viper" (also used metaphorically of a treacherous wife or friend), from ekhis "snake," from PIE *angwhi- "snake, eel" (source also of Norwegian igle, Old High German egala, German Egel "leech," Latin anguis "serpent, snake").

But this sense is difficult to reconcile with this animal (unless it is a reference to the ant-eating tongue). The name perhaps belongs to Latin echinus, Greek ekhinos "sea-urchin," originally "hedgehog" (in Greek also "sharp points"), which Watkins explains as "snake-eater," from ekhis "snake." The 1810 Encyclopaedia Britannica gives as the animal's alternative name "porcupine ant-eater." Or, more likely, the name refers to Echidna as the name of a serpent-nymph in Greek mythology, "a beautiful woman in the upper part of her body; but instead of legs and feet, she had from the waist downward, the form of a serpent," in which case the animal was so named for its mixed characteristics (early naturalists doubted whether it was mammal or amphibian).

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