The Collaborative International Dictionary
Substract \Sub*stract"\, v. t. [F. suostraire; L. subtus below
(from sub under) + trahere to draw. See Substract.]
To subtract; to withdraw. [Obs.]
--Barrow.
Douglas Harper's Etymology Dictionary
"to subtract," 1540s, "now illiterate" [OED], "An erroneous form of subtract, common in vulger use" [Century Dictionary], from Modern Latin substractus, past participle of substrahere, alternative form of subtrahere (see subtraction). Related: Subtracted; subtracting.
Wiktionary
vb. (context obsolete English) To subtract.
Usage examples of "substract".
When, therefore, these two kinds of experience are contrary, we have nothing to do but substract the one from the other, and embrace an opinion, either on one side or the other, with that assurance which arises from the remainder.
The clergyman knew someting of surgery, and he managed to substract the ball from my hip.