1600-10; < Latinpostumus last-born, born after the death of the father (in form a superlative of posterus; see posterior); post-classical spelling with h by association with humus ground, earth, as if referring to burial
1608, "born after the death of the originator" (author or father), from L.L. posthumus, from L. postumus "last, last-born," superlative of posterus "coming after, subsequent." Altered in L.L. by association with L. humare "to bury," suggesting death; the one born after the father's death obviously being the last.