'slacken' originates from Old English/Old Norse roots, specifically the words 'slæc' (Old English) and 'slakr' (Old Norse), where the root meant 'loose' or 'slack'. The verb was formed in English by adding the causative/adjectival suffix '-en' to the root.

'slacken' changed from Middle English 'slaken' (derived from Old English/Old Norse roots 'slæc'/'slakr') and eventually became the modern English verb 'slacken'.

Initially, it meant 'to be loose' or 'loose in quality/firmness', but over time it evolved into the current meanings of 'make loose' and 'reduce in intensity or speed'.