Wiktionary
hole card
n. 1 (context poker chiefly in the plural English) A playing card, dealt face down, that the holder need not reveal until the showdown 2 (context by extension English) Something held in reserve, or concealed, until it is useful
WordNet
hole card
n. any assets that are concealed until they can be used advantageously
(poker) a playing card dealt face down and not revealed until the showdown
Usage examples of "hole card".
But when you push the stakes up, the hole card starts to look bigger and bigger.
After a dozen rounds of betting, with your life's savings and car and house on the line, that hole card stands taller than Mount Everest.
I didn't care to show my hole card so quickly, but I suppose it can't be helped.