1540s, "contrary to nature, reason, or common sense," from Latin praeposterus "absurd, contrary to nature, inverted, perverted, in reverse order," literally "before-behind" (compare topsy-turvy,cart before the horse), from prae "before" (see pre-) + posterus "subsequent, coming after," from post "after" (see post-).
The sense gradually shaded into "foolish, ridiculous, stupid, absurd." The literal meaning "reversed in order or arrangement, having that last which ought to be first" (1550s) is now obsolete in English. In 17c. English also had a verb preposterate "to make preposterous, pervert, invert." Related: Preposterously; preposterousness.