hole card
n.
► A card in stud poker that is dealt face-down and that the holder is not obliged to reveal before the showdown.
► Informal. Something held in reserve until it can be used advantageously.

[TahlilGaran] American Dictionary ▲

a hole card
[American]
a secret advantage that is ready to use when you need it.
She still had one hole card to get out of police custody - a phone call to an influential friend.

[TahlilGaran] English Idioms Dictionary ▲