How Well Do You Know English Slang?
mid-15c., "born after the death of the originator" (author or father), from Late Latin posthumus, from Latin postumus "last, last-born," superlative of posterus "coming after, subsequent" (see posterior). Altered in Late Latin by association with Latin humare "to bury," suggesting death; the one born after the father's death obviously being the last. An Old English word for this was æfterboren, literally "after-born." Related: Posthumously.