Preposterous - definition: absurd, senseless, foolish, ridiculous Today's word seems to have lost it's common usage - although it is a fun word to incorporate into every day conversation. The words origin is thought to be medieval when belief in monstrous and fabulous creatures was commonplace. The word itself has both 'pre' (front) and 'post' (rear) within it. In relation to the fantastic animals it describes their having parts in the wrong order, such as animals that had heads and tails reversed or even heads are both ends, for example - the Amphisbaena.