The phrase "fly open" can be used to describe the state of clothing that is not properly fastened, specifically when referring to pants or shorts. There are a number of synonyms that can also be used to convey this meaning, such as "unzipped," "undone," "unfastened," "open," "loose," or "unbuttoned." Depending on the context, other phrases may also be appropriate, such as "carelessly dressed," "slipshod," or "haphazard." Regardless of the words chosen, the intended meaning remains the same: clothing that is not securely fastened, which can often be embarrassing or cause discomfort if left unaddressed.