"tool for digging," Old English spadu "spade," from Proto-Germanic *spadan (source also of Old Frisian spada "a spade," Middle Dutch spade "a sword," Old Saxon spado, Middle Low German spade, German Spaten), from PIE *spe-dh-, from root *spe- (2) "long, flat piece of wood" (source also of Greek spathe "wooden blade, paddle," Old English spon "chip of wood, splinter," Old Norse spann "shingle, chip;" see spoon (n.)).
"A spade differs from a two-handed shovel chiefly in the form and thickness of the blade" [Century Dictionary]. To call a spade a spade "use blunt language, call things by right names" (1540s) translates a Greek proverb (known to Lucian), ten skaphen skaphen legein "to call a bowl a bowl," but Erasmus mistook Greek skaphe "trough, bowl" for a derivative of the stem of skaptein "to dig," and the mistake has stuck [see OED].
black figure on playing cards," 1590s, probably from Italian spade, plural of spada "the ace of spades," literally "sword, spade," from Latin spatha "broad, flat weapon or tool," from Greek spathe "broad blade" (see spade (n.1)). Phrase in spades "in abundance" first recorded 1929 (Damon Runyon), probably from bridge, where spades are the highest-ranking suit.
The invitations to the musicale came sliding in by pairs and threes and spade flushes. [O.Henry, "Cabbages & Kings," 1904]
Derogatory meaning "black person" is 1928, from the color of the playing card symbol.