adder (n.)youdaoicibaDictYouDict[adder 词源字典]
Old English næddre "a snake, serpent, viper," from Proto-Germanic *nædro "a snake" (cognates: Old Norse naðra, Middle Dutch nadre, Old High German natra, German Natter, Gothic nadrs), from PIE root *netr- (cognates: Latin natrix "water snake," probably by folk-association with nare "to swim;" Old Irish nathir, Welsh neidr "adder").

The modern form represents a faulty separation 14c.-16c. into an adder, for which see also apron, auger, nickname, humble pie, umpire. Nedder is still a northern English dialect form. Folklore connection with deafness is via Psalm lviii:1-5. The adder is said to stop up its ears to avoid hearing the snake charmer called in to drive it away. Adderbolt (late 15c.) was a former name for "dragonfly."[adder etymology, adder origin, 英语词源]
charm (v.)youdaoicibaDictYouDict
c. 1300, "to recite or cast a magic spell," from Old French charmer (13c.) "to enchant, to fill (someone) with desire (for something); to protect, cure, treat; to maltreat, harm," from Late Latin carminare, from Latin carmen (see charm (n.)). In Old French used alike of magical and non-magical activity. In English, "to win over by treating pleasingly, delight" from mid-15c. Related: Charmed; charming. Charmed (short for I am charmed) as a conventional reply to a greeting or meeting is attested by 1825.
it (pron.)youdaoicibaDictYouDict
Old English hit, neuter nominative and accusative of third person singular pronoun, from Proto-Germanic demonstrative base *khi- (cognates: Old Frisian hit, Dutch het, Gothic hita "it"), from PIE *ko- "this" (see he). Used in place of any neuter noun, hence, as gender faded in Middle English, it took on the meaning "thing or animal spoken about before."
Whether the charmer sinner it or saint it,
If the folly grow romantic, I must paint it.
[Pope, "Moral Essays," 1735]
The h- was lost due to being in an unemphasized position, as in modern speech the h- in "give it to him," "ask her," "is only heard in the careful speech of the partially educated" [Weekley]. It "the sex act" is from 1610s; meaning "sex appeal (especially in a woman)" first attested 1904 in works of Rudyard Kipling, popularized 1927 as title of a book by Elinor Glyn, and by application of It Girl to silent-film star Clara Bow (1905-1965). In children's games, meaning "the one who must tag the others" is attested from 1842.
leech (n.2)youdaoicibaDictYouDict
obsolete for "physician," from Old English læce, probably from Old Danish læke, from Proto-Germanic *lekjaz "enchanter, one who speaks magic words; healer, physician" (cognates: 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- "to collect," with derivatives meaning "to speak" (see lecture (n.)).

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 17c., leech usually was applied only to veterinary practitioners. 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.
snake (n.)youdaoicibaDictYouDict
Old English snaca, from Proto-Germanic *snakon (cognates: Old Norse snakr "snake," Swedish snok, German Schnake "ring snake"), from PIE root *sneg- "to crawl, creeping thing" (cognates: Old Irish snaighim "to creep," Lithuanian snake "snail," Old High German snahhan "to creep"). In Modern English, gradually replacing serpent in popular use.

Traditionally applied to the British serpent, as distinguished from the poisonous adder. Meaning "treacherous person" first recorded 1580s (compare Old Church Slavonic gadu "reptile," gadinu "foul, hateful"). Applied from 17c. to various snake-like devices and appliances. Snakes! as an exclamation is from 1839.

Snake eyes in crap-shooting sense is from 1919. Snake-bitten "unlucky" is sports slang from 1957, from a literal sense, perhaps suggesting one doomed by being poisoned. The game of Snakes and Ladders is attested from 1907. Snake charmer is from 1813. Snake pit is from 1883, as a supposed primitive test of truth or courage; figurative sense is from 1941. Phrase snake in the grass is from Virgil's Latet anguis in herba [Ecl. III:93].