Posted by Andrew on July 03, 2003
Charles Mackay in _The Lost Beauties of the English Language_ suggests it derives from the word spaed, meaning castrated. So to call a spaed, a spaed is to call a eunuch, a eunuch.
I think it good palin English without fraud,
To call a spade a spade, a bawd a bawd.
Taylor, the Water Poet, 1630