I always assumed it was a poker saying about bluffing or cheating
but according to Wikipedia For instance, you may not want to call a spade a spade. You may prefer to call it a spatulous device for abrading the surface of the soil. Better, however, to stick to the old familiar, simple name that your grandfather called it for being racist. The phrase predates the use of the word "spade" as an ethnic slur against African Americans, which was not recorded until 1928; however, in contemporary U.S. society, the idiom is often avoided due to potential confusion with the slur[3] or confusion with playing card references such as "black as the ace of spades".