Green’s Dictionary of Slang

nip n.²

In phrases

in one’s nip (also in the nip) [? SE nip in the air, i.e. one suffers from the cold]

(Irish) stark naked.

1991		R. Doyle Van (1998) 429: Yeh’d be better off goin’ around in your nip, said Jimmy Sr.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Nip (n): nude, as in ‘I saw her in the nip’.
2019		Twitter 2 Apr. 🌐 Jumpsuits are all good fun until you’re sat on the toilet in the nip, with the thing around your ankles, helplessly watching as the electronic doors start to open.
2023		K. Lister Twitter 17 Mar. 🌐 On today’s episode of #BetwixtTheSheets, I am talking to the utterly amazing [...] completely naked @vnbateman [...] And just to really get into the spirit of it, myself & my producer were in the nip as well. Enjoy!