Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
holdup (adj., n.), hold up (v.)
The noun, meaning “a robbery at gunpoint or the like,” is a one-word compound. The verb, meaning “to conduct such a robbery,” as in He plans to hold up a bank, is always two words and is a combined form, not a compound. The adjective, as in the holdup man, is another one-word compound (and also is part of a second compound with man).
