What's the meaning of the word 'Preposterous'?
The word preposterous is now chiefly used to mean absurd; ridiculous, although it originally has a specific and literal meaning - see below.
What's the origin of the word 'Preposterous'?
I know that this site generally deals with phrases rather than single words, but this word packs in enough to be counted as virtually a phrase. We may go through life using 'preposterous' as a synonym for 'absurd; laughable; ridiculous' without giving any thought to why it means that.
By breaking preposterous into its constituent parts it becomes clear that its meaning can be read quite literally, that is, it describes something in which the 'pre' (front) has become the 'post' (rear). So, 'preposterous' things or, more usually animals, were those which were contrary to nature; having their parts in the wrong order.
A medieval belief in the existence of fabulous animals was commonplace, indeed almost universal. These were drawn and described in books called bestiaries. Many of these medieval books survive and contain numerous examples of the weird and wonderful.
To the usual collection of dragons, griffins, mermaids, unicorns and the like we can also add animals that had heads at both ends, for example the snake-like animal the Amphisbaena. Numerous representations of amphisbaenas exist - all of them different, as befits an invented creature.
The use of 'preposterous' to refer to things which were wrong or inverted dates from 1533, when it was so used several times in a translation of Erasmus' Enchiridion Militis Christiani. The use of the term to refer specifically to 'wrong' animals was in use by at least 1661, when it appeared in Joseph Glanvill's The Vanity of Dogmatizing:
"Thus our Eyes, like the preposterous Animal's, are behind us."
See also: 'Lick into shape'.