hold up vs holdup

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

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

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

hold up (verb)	holdup (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