What does Robert L Stevenson mean when he calls the woman in Chapter 3 of Kidnapped a "limmer rowpit"?

Answer 1

It's a vulgar Scottish term (now largely outdated) for a scoundrel, prostitute, or similar person of low repute.

Answer 2

A 'limmer' is a term for a vulgar woman, an old witch or a harridan. 'Roupit' means evicted and possessions sold. So what David's uncle is saying is that he, being her landlord, will have the old hag thrown out of her cottage.