Flounder is a relatively common verb that current evidence dates to the late 16th century, when it was likely born by means of an alteration of an older verb, founder. The two have been confused ever since. Today, founder is most often used as a synonym of fail, or, in the case of a waterborne vessel, as a word meaning "to fill with water and sink." Formerly, it was also frequently applied when a horse stumbled badly and was unable to keep walking. It's likely this sense of founder led to the original and now-obsolete meaning of flounder: "to stumble." In modern use, flounder typically means "to struggle" or "to act clumsily"; the word lacks the finality of founder, which usually denotes complete collapse or failure, as that of a sunken ship.