locum vs backup man

locum and backup man both are nouns.

Word	Noun	Adjective	Verb	Adverb
locum	Yes	No	No	No
backup man	Yes	No	No	No

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

locum: someone (physician or clergyman) who substitutes temporarily for another member of the same profession
backup man: someone who takes the place of another (as when things get dangerous or difficult)

Other hypernyms of locum include backup, fill-in, relief, reliever, stand-in, substitute.

locum (noun)	backup man (noun)
someone (physician or clergyman) who substitutes temporarily for another member of the same profession	someone who takes the place of another (as when things get dangerous or difficult)