mid-14c., passager "a passer-by; a traveler," from Old French passagier, passageor "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). The oldest sense now is obsolete; meaning "one traveling in a public vehicle or vessel," especially in exchange for a fare, is attested from 1510s; hence, in modern use, "one riding in a private vehicle driven by another." The railroad passenger-car is attested from 1832. The North American passenger-pigeon was so called from 1802 for its extensive wanderings in search of food; they have been extinct since 1914.