1590s, "to communicate with another or others in search of mutual agreement," a back-formation from negotiation, or else from Latin negotiatus, past participle of negotiari "carry on business, do business," from negotium "a business, employment, occupation, affair (public or private)," literally "lack of leisure," from neg- "not" (from PIE root *ne- "not") + otium "ease, leisure," a word of unknown etymology.
Transitive sense of "arrange for or procure by negotiation" is from 1610s. In the sense of "handle, manage, tackle successfully" (1862), it at first meant "to clear on horseback a hedge, fence, or other obstacle" and "originated in the hunting-field; those who hunt the fox like also to hunt jocular verbal novelties" [Gowers, 1965]. Related: Negotiated; negotiating.
It forms all or part of: a- (3) "not, without;" abnegate; ahimsa; an- (1) privative prefix; annihilate; annul; aught (n.2) "zero, nothing;" deny; hobnob; in- (1) "not, opposite of, without;" ixnay; naught; naughty; nay; nefarious; negate; neglect; negligee; negotiate; neither; nepenthe; nescience; nescient; neuter; never; nice; nihilism; nihility; nil; nill; nimiety; nix; no; non-; none; nonplus; nor; not; nothing; null; nullify; nulliparous; renegade; renege; un- (1) prefix of negation; willy-nilly.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit a-, an- "not;" Avestan na "not;" Greek a-, an-, ne- "not;" Latin in- "not," ne "that not;" Old Church Slavonic and Lithuanian ne "not;" Old Irish an-, ni, Cornish ny "not;" Gothic and Old English un- "not."
"invested with, having, or bestowing full power," 1640s, from French plénipotentiaire and directly from Medieval Latin plenipotentiarius "having full power," from Late Latin plenipotens, from Latin plenus "complete, full" (from PIE root *pele- (1) "to fill") + potentem "powerful" (see potent). As a noun from 1650s, "person invested with full powers to transact any business," especially with reference to an ambassador to a foreign court or government, given full power to negotiate a treaty or transact other business.
Meaning "to entertain with food and drink without expense to the recipient by way of compliment or kindness (or bribery)" is recorded from c. 1500. Sense of "deal with, handle, or develop in speech or writing" (early 14c.) led to the use in medicine "to attempt to heal or cure, to manage in the application of remedies" (1781). Related: Treated; treating.