reek vs niff

reek and niff both are nouns.

Word	Noun	Adjective	Verb	Adverb
reek	Yes	No	Yes	No
niff	Yes	No	No	No

As nouns, niff is a hyponym of reek; that is, niff is a word with a more specific, narrower meaning than reek:

Other hyponyms of reek include pong.

reek (noun)	niff (noun)
a distinctive odor that is offensively unpleasant	an unpleasant smell

reek (verb)	niff (verb)
be wet with sweat or blood, as of one's face
smell badly and offensively
have an element suggestive (of something)
give off smoke, fumes, warm vapour, steam, etc.