Green’s Dictionary of Slang

hijack n.

[ety unknown; virtually SE by 1900, as are the later meanings referring to the holding up of vehicles, including aircraft, and the killing or ransoming of their occupants]

1. (orig. US) a hold-up followed by the theft of goods (often exercised by one criminal upon another), thus the gangster/robber who performs the hold-up; also attrib.

[US]Coffeyville Dly Jrnl 1 Mar. 3/1: A Hi-Jack is a man who robs and thief of his plunder and appropriates it unto himself. Hi-jacking is an immoral practice and booze-haulers [...] have evidently combined in a crusade to stop the operations of the Hi-Jacks.
[US]Chanute Dly Trib. (KS) 2 Mar. 4/5: [headline] ‘Hi-Jacks’ Engage in Deadly Battle [...] Lee and Elder had attempted to pull off a ‘hi-jack’ job and capture for their own use an automobile load of booze.
[US]Detroit Free Press 1 Dec. 5/2: ‘I was a sucker [...] They told me there was easy money in the hijack game’.
[US]Goldin et al. DAUL 95/2: Hijack, n. The act of hijacking.

2. (US) intimidation, esp. by organized labour, those those who carry it out; also attrib.

[US]Orlando Eve. Star (FL) 8 Aug. 3/2: Many of the 146 witnesses [...] told of ‘Hi-Jack’ or string-arm methods of the oragnization [i.e. the I.W.W.] [...] ‘Hi-Jack’ tactics were also used to increase membership [...] ‘Scissor-Bills,’ for non-members, were thrown from trains, locked in sheds [etc].
[US]Chicago Trib. 31 May 17/7: [headline] I.W.W. Plot to Dope Soldiers Told at Trial. Frank Wormke was a ‘sab-cat’ and a ‘hi-jack’ until Uncle Sam got into the war .

3. (US tramp) the robbery of tramps as they sleep in the ‘hobo jungles’; thus the individual who does this.

[US]N. Anderson Hobo 24: Suddenly a commotion is started; a man is roughly rushed into the open. He is a hi-jack caught in the act of robbing a fellow who was sleeping, a greater crime in the jungle than an open hold-up.

4. (US tramp) a robber who uses violence.

[US]N. Anderson Hobo 102–3: The Jack Roller is a tramp who robs a fellow-tramp while he is drunk or asleep. There is a type of ‘Jack’ who operates among the men going to and from the harvests. He may hold them up in a box car with a gun or in some dark alley. He is usually called a Hi-Jack.