(Australia, Ireland, New-Zealand, UK, informal) A bad smell.
Police helicopters blared loudspeaker warnings about the smelly man. Soon afterwards, a nosy motel receptionist noticed a nasty niff from a new guest. 2014, Nick Arnold, Horrible Science: Body Owner's Handbook, page 159:
Whew, there's a bit of a niff in here. Is that your socks?
(Cornwall) A sulk or sullen mood.
'I don't see you need to get in such a niff,' said Katie. 2008, Winston Graham, The Twisted Sword, page 492: