Definition
A person, usually a young boy or teenager, who is particularly happy.
How People Actually Use It
"Chap" is an informal antiquated way for "fellow", "dude", "guy". It becomes "chappy" for making it rhymes with "happy". If you're happy chappy, you're pleased, happy. Often used in a sarcastic way and with a negative.
"- "Omg, how do you even like this song, it really sucks!" - "Sounds like you're not a happy chappy...go listening to your stuff then""
Source: Urban Dictionary