To the dismay of those few surviving writers on language who like to argue sense from etymology, this word no longer quite means what its Latin precursor says it ought. It appeared in English in the sixteenth century, derived from the classical Latin praeposterus, which is made up of prae, before, plus posterus, coming after, so meaning something reversed (and hence nonsensical).
In the middle of the nineteenth century it attracted the ire of the philologist Richard Chenevix Trench, who was at various times Dean of Westminster and Archbishop of Dublin, but is much better known to dictionary makers as the man who put forward the original idea for the Oxford English Dictionary.
He argued in his book English Past and Present that the term strictly refers to something that is absurd because the true order of things has been reversed: “It is ‘preposterous,’ in the most accurate use of the word, to put the cart before the horse, to expect wages before the work is done, to hang a man first and try him afterwards; and in this strict and accurate sense the word was always used by our elder writers”. He lamented that the word had been debased by sloppy writers to the point at which “It is now no longer of any practical service at all in the language, being merely an ungraceful and slipshod synonym for absurd”.
Dean Chenevix Trench may have been one of the founders of modern linguistics, but here he makes the same mistake as earlier writers on language who felt that words ought always to mean what their Latin originals meant and that change always implies decay. Sadly for him, his most famous inspiration, the OED, eventually refuted him. It records that the sense of something absurd was first used by Nicholas Udall in 1542, in his English translation of the works of Erasmus, ten years before the first example of the sense Trench regarded as the only proper one.