Posthumus Dutch, Low German
From 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.