loosen vs undo

loosen and undo both are verbs.

Word	Noun	Adjective	Verb	Adverb
loosen	No	No	Yes	No
undo	No	No	Yes	No

As verbs, loosen and undo are synonyms defined as:

Other synonyms of loosen include untie.

loosen (verb)	undo (verb)
become loose or looser or less tight	cancel, annul, or reverse an action or its effect
make loose or looser	cause to become loose
make less dense	remove the outer cover or wrapping of
disentangle and raise the fibers of	deprive of certain characteristics
cause to become loose	cause the ruin or downfall of
become less severe or strict
make less severe or strict