holdup man vs thief

holdup man and thief both are nouns.

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

As nouns, thief is a hypernym of holdup man; that is, thief is a word with a broader meaning than holdup man:

holdup man: an armed thief
thief: a criminal who takes property belonging to someone else with the intention of keeping it or selling it

Other hypernyms of holdup man include stealer.

holdup man (noun)	thief (noun)
an armed thief	a criminal who takes property belonging to someone else with the intention of keeping it or selling it