A fork of Rural Dictionary
Happy, sunny. Something that makes you happy is cheery, and people are cheery when they are in a good mood. Cheery is an old fashioned term, and is now used primarily in sarcastic comments, where the word "cheery" actually means "depressing."
As in: "I heard you whistling. You must be in a cheery mood." Or sarcastically: "Hearing about that suicide was definitely cheery."