- this is thought to be derived from Old Norse slakki, 'a hollow between hills', but some meanings of the word may show influence from Scots slock and its ancestor, Gaelic sloc, 'a pit, a den'.
Examples include Slackend in West Lothian and Gateslack in Dumfriesshire. In modern use, and in minor names and field-names, a slack can also be a low-lying waterlogged depression in the ground, typically a marsh or morass. Sir Walter Scott notes this usage in his novel, Guy Mannering (1815):
Between the farm-house and the hill-pasture was a deep morass, termed ... a slack.