To beat with or as if with a flail, especially as a punishment. See Synonyms at beat. To swing or strike in a manner suggesting the action of a flail: The alligator thrashed its tail. To defeat utterly; vanquish. To thresh. To sail (a boat) against opposing winds or tides. To move wildly or violently: thrashed about all night. To strike or flail. To thresh. To sail against opposing tides or winds. The act or an instance of thrashing. Music See speed metal. thrash out To discuss fully.