The Collaborative International Dictionary of English v.0.48:

Subtract \Sub*tract"\, v. t. [imp. & p. p. Subtracted; p. pr.
   & vb. n. Subtracting.] [L. subtractus, p. p. of subtrahere
   to draw from beneath, withdraw, remove; sub under + trahere
   to draw. See Trace, v. t., and cf. Substract.]
   To withdraw, or take away, as a part from the whole; to
   deduct; as, subtract 5 from 9, and the remainder is 4.
   [1913 Webster]