Posthumus Dutch, Low GermanFrom a personal name which was given to a posthumous child, i.e., one born after the death of his father, derived from Latin
postumus "last, last-born" (superlative of
posterus "coming after, subsequent") via Late Latin
posthumus, which was altered by association with Latin
humare "to bury", suggesting death (i.e., thought to consist of
post "after" and
humus "grave", hence "after death"); the one born after the father's death obviously being the last.