The ‘spades’ in this phrase refers to the highest suit in cards, not the shovel. How did this shape get its name?
Playing Cards originated in Asia and spread across Europe around the 14th century. It arrived in England a little later than in Spain, Italy and Germany.
“Essentially, the Italian versions of early cards used the suits Cups, Swords, Coins and Batons — which, on migration to England, became Hearts, Spades, Diamonds and Clubs. The image for Spades on English and French cards looks somewhat like that of the German Acorn or Leaf suits, but its origin is revealed by its name rather than its shape. The Spanish and Italian for sword is ‘espada’ and ‘spada’ respectively, hence the suit ‘Swords’ became anglicized as ‘Spades’.”
So where does the non-card-playing meaning come from? It is an Americanism:
First of all, the phrase isn’t found before the 1920s. Damon Runyon, an American journalist and writer, used the expression that way in a piece for Hearst’s International magazine, in October 1929:
“I always hear the same thing about every bum on Broadway, male and female, including some I know are bums, in spades, right from taw.“
Some other spade phrases: “cocky as the King of Spades”, “call a spade a spade”, “spade something up”