Definition from Wiktionary, the free dictionary
In most states in the United States, drinking age is 21 years and an ace in poker often has the value of one. Hence, one could say that a 2-A hand in poker could have a value equivalent to 2-1.
- Used other than as an idiom: The least age at which one is permitted by law to drink alcoholic beverages.
- (poker slang) A two and an ace as a starting hand in Texas hold 'em