late 13c., "command on oath;" c. 1300, "summon by a sacred name, invoke by incantation or magic," from Old French conjurer "invoke, conjure" (12c.) and directly from Latin coniurare "to swear together; conspire," from assimilated form of com "with, together" (see con-) + iurare "to swear," from ius (genitive iuris) "law, an oath" (see jurist).
The magical sense is from the notion of "constraining by spell" a demon to do one's bidding. Related: Conjured; conjuring. Phrase conjure up "cause to appear in the mind" (as if by magic) attested from 1580s.
"to conjure, to guess," originally "to make out by supernatural insight," mid-14c., divinen, from Old French deviner, from Vulgar Latin *devinare, a dissimilation of *divinare, from Latin divinus "of a god," from divus "of or belonging to a god, inspired, prophetic," related to deus "god, deity" (from PIE root *dyeu- "to shine," in derivatives "sky, heaven, god"). Latin divinus also meant, as a noun, "soothsayer." Related: Divined; divining. Divining rod (or wand) attested from 1650s.
"toilet," 1932, probably from jakes, used for "toilet" since 15c. Meaning "prostitute's customer" is from 1911, probably from the common, and thus anonymous, name by which they identified themselves. Meaning "policeman" is by 1901, from shortening of johndarm (1823), a jocular Englishing of gendarme.
"John Darm! who's he?" "What, don't you know!
In Paris he is all the go;
Like money here,—he's every thing;
A demigod—at least a king!
You cannot fight, you cannot drink,
Nor have a spree, nor hardly think,
For fear you should create a charm,
To conjure up the fiend John Darm!
["John Darm," in "Varieties in Verse," John Ogden, London, 1823]
"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.