Parlous is both a synonym and a derivative of perilous; it came to be as an alteration of perilous in Middle English. (Perilous is derived from the Anglo-French perilleus, which ultimately comes from the Latin word for "danger," perīculum.) Both words are documented in use from at least the 14th century. Seven centuries later, both remain in steady use, often modifying state or times; however, perilous is, by far, the preferred synonym. More than likely, this is from having the familiar peril as a base.