ace in the hole

ace in the hole ( hidden resource) — туз в рукаве; скрытое преимущество; козырь, оставленный про запас;

Example of a male speaking to a friend: "The real way to win a woman's heart is to bring her a little gift on your date. Don't let her know until about mid-way through the meal. That will be the real ace in your hole." (Samantha)

Related vocabulary
[arrow in one's quiver]
[туз в рукаве]
[two strings to one's bow]
ace up one's sleeve,
card up one's sleeve,
decisive argument, decisive resource, hidden advantage, reserve, secret advantage, secret weapon, sure advantage, trump card
a trump up one's sleeve