A held thing of value, kept because it will be needed. Named by its worth to a holder rather than by what it is.
Anglo-French asetz / Old French asez (enough, sufficient), from Vulgar Latin ad satis (to sufficiency). Originally a LEGAL term: an executor had "assets" when the estate held enough to settle the deceased's debts. The modern sense — a valuable thing owned — is a back-formation from that plural, and the singular "asset" does not appear in English until the 19th century.
A held thing of value, kept because it will be needed. The legal root is more honest than the accounting one: an asset was originally about SUFFICIENCY — having enough to meet what is owed — rather than about accumulation.