Yours, Etc.: Origins and Uses of 8 Sign-Offs
early 15c., "stab with a sword, kill," also "remove the ovaries of," from Anglo-French espeier "cut with a sword," from Middle French espeer, from Old French espee "sword" (French épée), from Latin spatha "broad, flat weapon or tool," from Greek spathe "broad blade" (see spade (n.1)).
v. spayed, spay·ing, spays
To surgically remove the ovaries of an animal.