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When a dog wags his or her tail, hence their butt is happy.
I could tell my dog was happy to see me because she had a happy butt.
by happybuttdog January 09, 2011
When things are going well for you.
An extra little bounce in your step.
A really good mood.
Sally: "You're in a good mood today!"
Dave: "I woke up with a good dose of happy butt!"
by jsd96321 January 12, 2012