Green’s Dictionary of Slang

muff v.3

[? muff n.1 (7) or muff v.1 (1)]

(UK juv.) to fart.

OnLine Dict. of Playground Sl. 🌐 muffy n. to break wind, fart, e.g. ‘Who let Muffy off the chain?’ exclaimed after an anonymous fart to discover the perpetrator. Sometimes abbreviated to ‘Who muffed?’.