early 15c., "to take away, separate, or remove in estimating or counting," from Latin deductus, past participle of deducere "lead down, bring away;" see deduce, with which it formerly was interchangeable. Deduct refers to taking away portions or amounts; subtract to taking away numbers. Related: Deducted; deducting.
Deduct is to lead away, set aside, in a general or distributive sense; subtract, to draw off, remove, in a literal or collective sense. In settling a mercantile account, certain items, as charges, losses, etc. are deducted by being added together and their total subtracted from the grand total of the transaction. [Century Dictionary]