Slacken comes from slack (from Old English slæc 'loose, lazy', of Germanic origin) plus the verb-forming suffix -en. The historical chain includes Old English slæc. The quoted older sense, "loose, lazy," supplies the immediate semantic anchor for slacken. The formation is best read as suffixal and borrowed: the word slacken carries its older form-history into a compact English lexical shape. In present English it is used as a verb, "to make or become slower, looser, or less intense". Related English forms include slack, slackness, and slacker. The result is a verb whose etymology preserves both the older form and the later English specialization.