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.
"simpleton, fool," 1590s, perhaps a misdivision of an innocent (see N for other examples), or from the pet form of the proper name Innocent, with sense influenced by the name's literal meaning. There may be some influence in the word of Italian ninno "baby, child" (cognate with Spanish niño). Related: Niniversity "school for idiots" (1580s).
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit nasyati "disappears, perishes," Avestan nasyeiti "disappears," nasu- "corpse," Old Persian vi-nathayatiy "he injures;" Greek nekros "corpse;" Latin nex, genitive necis "violent death, murder" (as opposed to mors), nocere "to harm, hurt," noxius "harmful;" Greek nekus "dead" (adj.), nekros "dead body, corpse;" Old Irish ec, Breton ankou, Welsh angeu "death."
late 13c., "foolish, ignorant, frivolous, senseless," from Old French nice (12c.) "careless, clumsy; weak; poor, needy; simple, stupid, silly, foolish," from Latin nescius "ignorant, unaware," literally "not-knowing," from ne- "not" (from PIE root *ne- "not") + stem of scire "to know" (see science). "The sense development has been extraordinary, even for an adj." [Weekley] -- from "timid, faint-hearted" (pre-1300); to "fussy, fastidious" (late 14c.); to "dainty, delicate" (c. 1400); to "precise, careful" (1500s, preserved in such terms as a nice distinction and nice and early); to "agreeable, delightful" (1769); to "kind, thoughtful" (1830).
In many examples from the 16th and 17th centuries it is difficult to say in what particular sense the writer intended it to be taken. [OED]
By 1926, it was pronounced "too great a favorite with the ladies, who have charmed out of it all its individuality and converted it into a mere diffuser of vague and mild agreeableness." [Fowler]
"I am sure," cried Catherine, "I did not mean to say anything wrong; but it is a nice book, and why should I not call it so?" "Very true," said Henry, "and this is a very nice day, and we are taking a very nice walk; and you are two very nice young ladies. Oh! It is a very nice word indeed! It does for everything." [Jane Austen, "Northanger Abbey," 1803]