To worry, bother, or trouble someone; to cause annoyance or distress (primarily Scottish and Northern English dialect).
From Middle English, possibly from Old French 'fachier' or Scandinavian roots. The word has remained strong in Scottish English while becoming archaic in standard English.
'Dinnae fash yerself' (don't worry yourself) is still used in Scottish English, and the word shows how regional dialects preserve old words that have nearly died out in other varieties of English—'fash' has been around for 700 years.
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