1
general::
1. Beneficial to, as in Milk is good for children. [Tenth century A.D.] 2. Financially reliable, able to pay or repay, as in They know he's good for a big tip. [Mid-1800s] 3. Able to serve or continue to function, as in This furniture's good for at least ten more years, or I hope you're not tired? I'm good for another three miles or so. [Mid-1800s] 4. Equivalent in value; also, valid for. For example, These cou pons are good for a 20 percent discount, or This contract is good for the entire life of the book. [Second half of 1800s] 5. good for someone. An expression of approval, as in Good for Bill? he's sold the car, or Good for you! You passed the exam. This usage differs from the others in that orally a slight emphasis is placed on you or whoever is being mentioned. [Mid-1800s]
American Heritage Idioms