1590s, from Anglo-French escrowe, from Old French escroue "scrap, roll of parchment," from a Germanic source akin to Old High German scrot "a scrap, shred, a piece cut off" (see shred (n.)). Originally "a deed delivered to a third person until a future condition is satisfied;" sense of "deposit held in trust or security" is from 1888.
The condition of being ineffective until certain conditions are met. For example, money inherited by a minor might be held in escrow until the heir reaches a certain age. Homeowners with mortgages frequently pay money for insurance and taxes on their home into an escrow account each month. The holder of the mortgage then pays the insurance and tax bills out of the escrow account when the bills are due.