To be pleasant and in good spirits. I'm a happy camper now that I've been promoted and am no longer an intern!He's not going to be a happy camper when he finds out you ate his leftovers.
A satisfied participant, a contented person, as in She loved the challenge of her new job; she was one happy camper. This expression is also often put in the negative, as in She hated the heat and humidity of the southern summer; she was not a happy camper. [Slang; mid-1900s]