good for

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 coupons 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]