If you unfasten something that is closed, tied, or held together, or if you unfasten the thing holding it, you loosen or remove the thing holding it. When Ted was six we decided that he needed to know how to fasten and unfasten his seat belt Reaching down, he unfastened the latch on the gate He once emerged from the toilets with his flies unfastened. = undo fasten. to undo something such as a button, belt, rope etc