The antonym of the word "Fasten" is -

• Fasten (Verb).
- English meaning: To close or join together the two parts of something; to become closed or joined together.
- Bangla meaning: দৃঢ়বদ্ধ করা বা হওয়া; আটকানো; জড়ানো।

• Synonym: Attach, Affix, Tie, Glue.
• Antonym: Unfasten, Detach, Undo, Unhook.

• উল্লিখিত অপশনগুলো,
- Guise - বাহ্য রূপ, ছল, আকার;
- Wise - জ্ঞানী;
- Affix - যুক্ত/ সংযুক্ত করা;
- Dissociate - পৃথক করা, সংশ্লিষ্ট না-থাকা।

• Example sentence:
- Please fasten your seatbelt before the plane takes off.
- She fastened the buttons of her coat tightly.

Source: Merriam & Webster Dictionary, Oxford Dictionary, Accessible Dictionary.