1. To become loose or looser; SYN. relax.
2. To make less dense.
3. To make loose or looser; SYN. loose.

To untie or unfasten

1. To remove a band from; free from fastenings; untie, unfasten
2. To set free; release

ETYM 1 st pref. un- + hitch.
To unfasten or release from or as if from a hitch.

To separate or undo, as links; to uncoil; to unfasten.

ETYM as. untygan. Related to Un-, and Tie.
To undo the ties of; SYN. unbrace, unlace.