holdup vs hold up

holdup is a noun but hold up is not a noun.

Word	Noun	Adjective	Verb	Adverb
holdup	Yes	No	No	No
hold up	No	No	Yes	No

holdup (noun)	hold up (noun)
robbery at gunpoint
the act of delaying; inactivity resulting in something being put off until a later time

holdup (verb)	hold up (verb)
	cause to be slowed down or delayed
	be the physical support of; carry the weight of
	hold up something as an example; hold up one's achievements for admiration
	rob at gunpoint or by means of some other threat
	continue to live and avoid dying
	resist or withstand wear, criticism, etc.
	resist or confront with resistance