by 1922 (perhaps c. 1918), American English, of unknown origin; perhaps from high(way) + jacker "one who holds up" (agent noun from jack (v.)). In early use "to rob (a bootlegger, smuggler, etc.) in transit;" sense of "seize an aircraft in flight" is 1968 (also in 1961 variant skyjack), extended 1970s to any form of public transportation. Related: Hijacked; hijacking. Related: Hijacker.

"to hijack an airplane," by August 1961 in U.S. newspapers, apparently coined in headlines in New York Mirror and others, from sky (n.) + second element abstracted from hijack (q.v.).

From hijack to skyjack is no more than a twitch of a headline writer's pencil, and thus America is introduced to a new colloquialism that is thoroughly understood. For general use, however, the word hijack colorfully persists, and skyjack is likely to enjoy ephemeral usage. [editorial, Stockton (Calif.) Record, Aug. 21, 1961]

Related: Skyjacked; skyjacking; skyjacker (1961).