holdup man vs hijacker

holdup man and hijacker both are nouns.

Word	Noun	Adjective	Verb	Adverb
holdup man	Yes	No	No	No
hijacker	Yes	No	No	No

As nouns, hijacker is a hyponym of holdup man; that is, hijacker is a word with a more specific, narrower meaning than holdup man:

Other hyponyms of holdup man include highjacker, highwayman, road agent.

holdup man (noun)	hijacker (noun)
an armed thief	a holdup man who stops a vehicle and steals from it
	someone who uses force to take over a vehicle (especially an airplane) in order to reach an alternative destination