mid-14c., "doing no evil; free from sin, guilt, or moral wrong," from Old French inocent "harmless; not guilty; pure" (12c.), from Latin innocentem (nominative innocens) "not guilty, blameless; harmless; disinterested," from in- "not" (see in- (1)) + nocentem (nominative nocens), present participle of nocere "to harm," from *nok-s-, suffixed form of PIE root *nek- (1) "death."

Meaning "free from guilt of a specific crime or charge" is from late 14c., as is the meaning "with childlike simplicity or artlessness." Humorous sense "free, devoid of" is from 1706. The noun meaning "person who is innocent of sin or evil, artless or simple person" is from c. 1200, especially a young child (who presumably has not yet sinned actively). The Holy Innocents (early 14c.) were the young children slain by Herod after the birth of Jesus (Matthew ii.16), hence Innocents day (Dec. 28).

Indo-European words for "innocent" are generally negative compound of the word for "guilty." An exception is the Germanic group represented by Gothic swikns (also "pure, chaste"), Old Norse sykn "free from guilt, innocent" (especially as a law term), Old English swicn "clearance from a charge," also "cleansing," but these are of uncertain origin.

"harmless, producing no ill effect, incapable of harm or mischief," 

1590s, from Latin innocuus "harmless; innocent; inoffensive," from in- "not" (see in- (1)) + nocuus "hurtful," from root of nocere "to injure, harm," from *nok-s-, suffixed form of PIE root *nek- (1) "death." Related: Innocuously; innocuousness.

1660s, "deadly, destructive," from Latin internecinus "very deadly, murderous, destructive," from internecare "kill or destroy," from inter (see inter-) + necare "kill" (from PIE root *nek- (1) "death").

Considered by OED as misinterpreted in Johnson's Dictionary [1755], which defined it as "endeavouring mutual destruction," but a notion of "mutually destructive" has been imported into the word in English because in English inter- usually conveys the idea of "mutual." The Latin prefix is said to have had here only an intensive sense; "the Latin word meant merely of or to extermination ... without implying that of both parties" [Fowler].

c. 1300, nygromauncy, nigromauncie, "sorcery, witchcraft, black magic," properly "divination by communication with the dead," from Old French nigromancie "magic, necromancy, witchcraft, sorcery," from Medieval Latin nigromantia (13c.), from Latin necromantia "divination from an exhumed corpse," from Greek nekromanteia, from nekros "dead body" (from PIE root *nek- (1) "death") + manteia "divination, oracle," from manteuesthai "to prophesy," from mantis "one who divines, a seer, prophet; one touched by divine madness," from mainesthai "be inspired," which is related to menos "passion, spirit" (see mania). The spelling was influenced in Medieval Latin by niger "black," perhaps on notion of "black arts" although in Latin the word also was used to signify death and misfortune. The modern English spelling is a mid-16c. correction. Related: Necromantic.

"large cemetery" of an ancient or modern city, 1803, from Late Latin, literally "city of the dead," from Greek Nekropolis, a burial place near Alexandria, from nekros "corpse" (from PIE root *nek- (1) "death") + polis "city" (see polis).

"death of bodily tissue," 1660s, from Latinized form of Greek nekrosis "a becoming dead, state of death," from nekroun "make dead," from nekros "dead body" (from PIE root *nek- (1) "death"). Related: Necrotic.

1550s, from Latin nectar, from Greek nektar, name of the drink of the gods, which is perhaps an ancient Indo-European poetic compound of nek- "death" (from PIE root *nek- (1) "death") + -tar "overcoming," from PIE root *tere- (2) "cross over, pass through, overcome." Sense of "any delicious drink" is from 1580s. Meaning "sweet liquid in flowers" first recorded c. 1600.

type of peach with smooth skin and firmer pulp, 1660s, noun use of adjective meaning "of or like nectar" (1610s; see nectar + -ine (1)). Probably inspired by German nektarpfirsich "nectar-peach." Earlier in English as nectrine.

of pain, "caused by potentially harmful stimuli," 1904, from Latin noci-, combining form of nocere "to do harm" (from PIE root *nek- (1) "death") + second element from receptive.

1630s, "noxious, harmful," from Latin nocuus "harmful," from stem of nocere "to hurt, injure, harm" (from PIE root *nek- (1) "death"). Especially of venomous serpents.