sandman vs imp

sandman and imp both are nouns.

Word	Noun	Adjective	Verb	Adverb
sandman	Yes	No	No	No
imp	Yes	No	No	No

As nouns, imp is a hypernym of sandman; that is, imp is a word with a broader meaning than sandman:

sandman: an elf in fairy stories who sprinkles sand in children's eyes to make them sleepy
imp: (folklore) fairies that are somewhat mischievous

Other hypernyms of sandman include brownie, elf, gremlin, hob, pixie, pixy.

sandman (noun)	imp (noun)
an elf in fairy stories who sprinkles sand in children's eyes to make them sleepy	one who is playfully mischievous
	(folklore) fairies that are somewhat mischievous