negotiator (n.)

early 15c., negotiacioun, "a dealing with people, trafficking," from Old French negociacion "business, trade," and directly from Latin negotiationem (nominative negotiatio) "business, traffic," noun of action from past participle stem of negotiari "carry on business, do business, act as a banker," from negotium "a business, employment, occupation, affair (public or private)," also "difficulty, pains, trouble, labor," literally "lack of leisure," from neg- "not" (from PIE root *ne- "not") + otium "ease, leisure," a word of unknown origin.

The sense expansion from "doing business" to also include "bargaining" about anything took place in Latin. Meaning "mutual discussion and arrangement of the terms of a transaction or agreement" is from 1570s.