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1. A standard expression of good wishes said to someone on their birthday. In this usage, both words are typically capitalized. A: "Happy Birthday, Tommy!" B: "Thank you everyone!" A: "Thank you for the wonderful present, Mom!" B: "You're welcome, sweetie. Happy Birthday!"
2. Used as a noun to mean an enjoyable birthday. Don't forget to wish your cousin a happy birthday today. Goodbye, dear! I hope you have a happy birthday today!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.