Green’s Dictionary of Slang

1. smelly, malodorous .

1898		W.A. Sun. Times (Perth) 2 Jan. 2/5: The ill-omened trio [...] almost invariably hail from ‘niffy’ Naples.
1901		Boy’s Own Paper 24 Aug. 751: Throw the niffy bull’s-eye out of the window.
1934		Wodehouse Right Ho, Jeeves 4: He kept them in his study in a kind of glass-tank arrangement, and pretty niffy the whole thing was.
1940		M. Marples Public School Slang 160: SMELL. Some forceful synonym is generally preferred to the simple word — e.g. hum, niff (adj. niffy), pong, stink, whiff [ad], whiffy).
1952		Bishop & Cunningham Unknown Nepal 39: The mountain air was niffy with the acrid smell of un-washed bodies.
1977		(con. 1941) R. Beilby Gunner 165: Bit niffy, innit.
1978		F. Norman Dead Butler Caper 137: Something was pretty niffy in the state of Denmark.
2006		J. Perrin Jim Perrin 5: By then, after 14 years on the road, formalin or no formalin it’s getting a bit niffy.
2023		R. Milward Man-Eating Typewriter 382: [W]as this niffy pit truly Heaven?

2. of food, rotten, ‘off’.

1920

St. John Ervine Foolish Lovers 156: It ain’t as niffy as it smells!

3. in fig. use, dubious, questionable.

1945

Long & Malcolm In Smuts’s Camp 83: Though twice out of three times he gets on to a false scent, the third time he is usually on to something a bit niffy.