praeposterus

Preposterous comes from the Latin praeposterus (reversed, inverted, absurd — from prae, 'before,' and posterus, 'coming after'). Something preposterous has its front and back swapped — the first thing last, the last thing first. The word entered English in the mid-sixteenth century. Its original meaning was precisely spatial: physically reversed, back-to-front. A preposterous arrangement was one where the order was wrong.

The shift from 'backwards' to 'absurd' happened quickly. If something is backwards, it does not work. If it does not work, it is foolish. If it is foolish, it is ridiculous. The spatial metaphor became a logical one within a century. By the late 1500s, Shakespeare used preposterous to mean both 'reversed' and 'absurd' — sometimes in the same sentence.

The word contains its own English echoes. 'Pre-' means before. 'Posterior' means the back. Pre-posterior: the front in the back. The word is almost onomatopoetic — it sounds ridiculous because it describes ridiculousness. Its four syllables have a lumbering, overblown quality that matches its meaning. Preposterous is a preposterous-sounding word.

Modern English uses preposterous almost exclusively for strong emphasis: an idea so wrong it is laughable. The spatial meaning has completely vanished. Nobody says 'that painting is preposterous' to mean it is hanging upside down. The word has been freed from its literal meaning and now lives entirely in the realm of indignant disbelief.