Happy is a feeling of joy, pleasure, or good fortune — exactly how you'd feel if you learned that you won the lottery or got accepted into your number one choice of colleges.
Happy hails from the Middle English word hap, meaning "good luck." Many of the early European words for happy actually referred to good luck, rather than a feeling of joy. On its own, happy means an enjoyable or satisfied state of being. When hyphenated, it can refer to an overwhelming inclination to do something. Someone who is trigger-happy, for example, has an overwhelming urge to shoot things, while a clothes-happy person is obsessed with taking trips to the mall.