early 14c., passager "passer-by," from Old French passagier "traveler, passer-by" (Modern French passager), noun use of passagier (adj.) "passing, fleeting, traveling," from passage "mountain pass, passage" (11c.), from passer "to go by," from Vulgar Latin *passare "to step, walk, pass," from Latin passus "step, pace" (from PIE root *pete- "to spread").
And in this I resemble the Lappwing, who fearing hir young ones to be destroyed by passengers, flyeth with a false cry farre from their nestes, making those that looke for them seeke where they are not .... [John Lyly, "Euphues and His England," 1580]
The -n- was added early 15c. (compare messenger, harbinger, scavenger, porringer). Meaning "one traveling in a vehicle or vessel" first attested 1510s. Passenger-pigeon of North America so called from 1802; extinct since 1914.