IPA: /ˈslækli/
KK: /slækli/
In a loose or relaxed way, without tension or firmness.
The rope hung slackly from the tree branch.
Slackly → It is formed from "slack" (from Old Norse *slakkr*, meaning loose or relaxed) and "-ly" (a suffix meaning in a manner of). The word "slackly" means in a loose or relaxed manner.
Think of something being 'loose' ('slack') and the '-ly' indicating the manner of doing it. This helps you remember that 'slackly' means to do something in a loose or relaxed way.