The following extract from BBC blog gives an interesting range of possible regional usages of the term, sometime used also as a verb.
It's a regional usage. I've heard it in various parts of the north of England and up in Scotland too. It has a whole range of meanings.
- When you hear somebody say 'my torch is all manky', it means it's not working properly, it's worthless, it's defective, it's a bit inferior. And then, I've heard people say 'oh, he's got manky socks' or 'that baby's nappy is manky' and that means it's grimy or dirty, it might even be a bit smelly - that's another usage of the term - 'bananas are manky' or 'that dustbin is manky' - it means it's rotten, it's disgusting, it's got a smell of some kind.
And it generalises from there, too, into personal feelings - if you're feeling under the weather for instance, in some parts of the country, you can say, you know, 'I feel manky today'.
And then up in Scotland especially, it's used quite nastily, as a term of abuse - you know, somebody might say 'you manky so and so', that's really quite harsh.
Very unclear origins. It certainly goes back to the 1950s, maybe before. Some people think it comes from an old French word meaning 'impaired'. I don't know. Certainly, it's been used in a wide range of constructions now.
- I've heard it used in the phrase 'you're going to mank it up' meaning 'you're going to mess it up'. I've actually heard somebody talk about something being 'mankified'!
To mankify - a verb. And then there's that northern dialect use in Yorkshire where somebody says 'I'm feeling manky', 'I'm feeling rough and unwell'. That sort of use so upset people, doctors in particular who didn't understand what it meant, that they actually decided to write a guide to Yorkshire dialect to help them out, and so there you get in this list of medical terms 'manky', feeling rough.