mid-15c., posthumus, "born after the death of the originator" (author or father), from Late Latin posthumus, from Latin postumus "last," especially "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 is in the ground obviously being his last. An Old English word for this was æfterboren, literally "after-born." Related: Posthumously.
"remaining after deductions," early 15c., from earlier sense of "trim, elegant, clean, neat" (c. 1300), from Old French net, nette "clean, pure, unadulterated," from Latin nitere "to shine, look bright, glitter" (see neat (adj.)). Meaning influenced by Italian netto "remaining after deductions." As a noun, "what remains after deductions," by 1910. The notion is "clear of anything extraneous."
Net profit is "what remains as the clear gain of any business adventure, after deducting the capital invested in the business, the expenses incurred in its management, and the losses sustained by its operation" [Century Dictionary]. Net weight is the weight of merchandise after allowance has been made for casks, bags, cases, or other containers.