The etymology of the word posthumous tells a complex story. In Latin, posterus is an adjective meaning "coming after" (from post, meaning "after"). The comparative form of posterus is posterior, and its superlative form is postumus, which means, among other things, "last." Postumus had specific application in referring to the last of a man's children, which in some cases meant those born after he had died. Latin speakers incorrectly identified the -umus in this word with humus, meaning "dirt" or "earth" (suggesting the ground in which the unfortunate father now lay). The Latin spelling became posthumus, as if the word were formed from post and humus, and both the "h" and the suggestion of "after burial" or "after death" carried over into English.

Where does posthumous come from?

Readers who are looking for the origins of the word posthumous may be interested to know that it is an example of a folk etymology. A folk etymology involves the respelling or changing of an unfamiliar word (often one borrowed from another language) to make it resemble an unrelated but better-known word or words. The classic example in English is cockroach, which was formed by substituting the English words cock (the name of a bird) and roach (the name of a fish) for the similar-sounding but totally unrelated Spanish word for a bug, cucaracha.
Posthumous comes from the Latin posthumus, which is itself an alteration of postumus ("born after the father's death"). It is thought that the word humus (meaning "dirt, earth" in Latin) was substituted for -umus in the mistaken belief that the word's final element had something to do with the soil in a grave.