Green’s Dictionary of Slang

hole card n.

[poker jargon hole card; the card that, in five-card stud, is dealt face down]

(orig. US) a secret, which can be either a weakness that, once discovered, can be exploited, or a hidden strength.

[UK]C.M. Russell Trails Plowed Under 71: Ye know ye can’t tell what an Injun’s got for a hole-card by readin’ his countenance; winner or loser he looks the same.
[US]D. Runyon ‘The Snatching of Bookie Bob’ in Runyon on Broadway (1954) 123: Bookie Bob’s hole card is his ever-loving wife’s opinion of him.
[US]D. Runyon Runyon à la Carte 28: He knows my hole card is my knowledge of his background in junk.
[UK]N. Nye Long Run (1983) 122: I’ll look at your hole card.
[US]K. Brasselle Cannibals 84: A rat-bastard-shit-heel [...] could never hurt you if you knew his hole card.
[US]E. Bunker Animal Factory 82: He wouldn’t have put his hand under his clothes even if he was [armed]; that showed the hole card.
[US](con. c.1967) J. Ferrandino Firefight 163: He done peeped your hole card.
[US]N. McCall Makes Me Wanna Holler (1995) 283: I knew I had a hole card: abortion.
[US]J. Lerner You Got Nothing Coming 196: Conversational foreplay over, Kansas is turning over his hole card.

In phrases

peep someone’s/one’s hole card (v.) [peep v.3 (1)]

(US prison/black) to work out a person’s (or consider one’s own) hidden attitudes and emotions.

[US] ‘Mexicana Rose’ in D. Wepman et al. Life (1976) 39: I peeped your hole card, you’re a funny-time lame.
[US]C. Himes Crazy Kill 55: Everybody’s just peeping at their hole cards now.
[US]N. Heard Howard Street 74: I done peeped you old cats’ hole card.
[US]R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].
[US]‘Master Pimp’ Pimp’s Rap 69: You don’t want these niggers to peep your hole card, do you?