stickup man vs hijacker

stickup man and hijacker both are nouns.

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

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

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

stickup 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