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c.1500, "trade, commerce," from Middle French trafique (mid-15c.), from Italian traffico (early 14c.), from trafficare "carry on trade," of uncertain origin, perhaps from a Vulgar Latin *transfricare "to rub across" (from Latin trans- "across" + fricare "to rub"), with the original sense of the Italian verb being "touch repeatedly, handle."
Or the second element may be an unexplained alteration of Latin facere "to make, do." Klein suggests ultimate derivation of the Italian word from Arabic tafriq "distribution." Meaning "people and vehicles coming and going" first recorded 1825. Traffic jam is 1917, ousting earlier traffic block (1895).
1540s, from traffic (n.) and preserving the original commercial sense. Related: Trafficked; trafficking. The -k- is inserted to preserve the "k" sound of -c- before a suffix beginning in -i-, -y-, or -e- (cf. picnic/picnicking, panic/panicky, shellac/shellacked).