Is it ensure, insure, or assure?
1922, American English, perhaps from high(way) + jacker "one who holds up." Originally "to rob (a bootlegger, smuggler, etc.) in transit;" sense of "seizing an aircraft in flight" is 1968 (also in 1961 variant skyjack), extended 1970s to any form of public transportation. Related: Hijacked; hijacking.
[origin uncertain; said to be fr the command High, Jack, telling a robbery victim to raise his hands; an early 1900s hobo sense, ''traveling hold-up man,'' is attested, which suggests that the source may be railroad and hobo slang; said to have originated in the California wheat fields and among the Wobblies; the name of the 1875 skit High Jack the Heeler is interesting but probably coincidental]