Goodness. Our use of skanky and manky is extremely mild in comparison with the examples that have been proposed. In the same line as JMH, I'd say that potted plants start looking rather manky if you don't water them for ten days. And my boots might be skanky after a walk in a muddy area, for instance... A person wouldn't be manky... but yes, you could say someone is skanky if s/he looks unkempt, I guess.

The musical connection is interesting, and I do realise now that I am familiar with that Marley song "Easy Skanking"... thanks wow.

In any case, I'm glad to see more people know and use these words. And I still find them beautifully expressive, even if semantically they denote something not too pleasant!