Green’s Dictionary of Slang

(UK Black) a plain-clothes, undercover police officer.

2017		67 ‘Waps’ 🎵 Summer '16 I was ducking from the undies and THG vans / No face, no case, no way these Jakes can’t prove who I am.
2018		Unknown T ‘Mad about Bars’ 🎵 We made the Tridents come / And dis one look like an undie.