1. Not closed or secured
   "the car door was unfastened"; "unfastened seatbelts"
    
2. Affording unobstructed entrance and exit; not shut or closed
   "an unfastened door";
   - open
    
3. Not buttoned
   "the wind picked up the hem of her unfastened coat";
   - unbuttoned
    
4. Not tied
   "The unfastened seat belt couldn't protect him during the crash";
   - untied

1. Become undone or untied
   "The shoelaces unfastened"
    
2. Cause to become undone
   "unfasten your belt"

See also: ajar, open, open-collared, unbarred, unbolted, undone, unlatched, unlocked, unsecured, wide-open

Type of: open, open up, undo

Antonym: fasten, fastened