Besides “sassanach”, there were also “lochlanachs” which were people from the Lake District. They were more ethnically Britonic than the lowland Scots who were most certainly Angles, and Saxons, and Jutes.
My ancestors were highland Scots and Irish. My grandfather and great grandfather both called city people Lowlanders as far back as I can remember. My Irish grandmother called mountain people Uplanders. 😁
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This definition from dictionary.com is the one that I have always encountered for “sassanach”. And it’s been my understanding that it is always, not often, meant disparagingly:
noun Often Disparaging. a term used by the Gaelic inhabitants of the British Isles to refer to the English inhabitants.