More idioms/slang terms

In “Tir Dial” and “Bedydd Tan”, Dyfed Edwards uses the terms slanu, slanwr and lefran a lot.
When I looked up “slanu” and “slanwr”, i got “heal” and “healer”. This makes no sense in the context of the novels, where it implies some sort of violent action.
Also, “lefran” is used to mean “girlfriend” but is this a patronising term, something like “bird” or “chick”, or is it more neutral?
As ever, thanks in advance

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Sorry, not heard any of these! What area was it set in?