undo vs unfasten

undo and unfasten both are verbs.

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

As verbs, unfasten is a hyponym of undo; that is, unfasten is a word with a more specific, narrower meaning than undo:

undo (verb)	unfasten (verb)
cancel, annul, or reverse an action or its effect	become undone or untied
cause to become loose	cause to become undone
remove the outer cover or wrapping of
deprive of certain characteristics
cause the ruin or downfall of