Unbuckle is a verb meaning to LOOSEN or unfasten something that has been secured with a buckle. It is often used in reference to seatbelts, straps, and other items that are held together with a buckle. Unbuckle is made up of two words, un- and buckle, which both have their own meanings. Un- means to undo or reverse an action, while buckle means to fasten or secure something. Together, they form the word unbuckle, which means to undo or unfasten something that has been secured with a buckle.