Green’s Dictionary of Slang

hole card n.

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

[1920s+] (orig. US) a secret, which can be either a weakness that, once discovered, can be exploited, or a hidden strength.

In phrases

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

[1950s+] (US prison/black) to work out a person’s (or consider one’s own) hidden attitudes and emotions.