NIFF

niff, pong

(noun) an unpleasant smell

Source: WordNet® 3.1

Noun

niff (plural niffs)

(informal) A bad smell.

Verb

niff (third-person singular simple present niffs, present participle niffing, simple past and past participle niffed)

(informal) To smell bad; to stink.

Source: Wiktionary