Feeling extremely happy or joyful.
It's a very strong feeling of happiness, like when something really good happens.
Examples:
She was the slaphappiest person at the party.
The kids were slaphappy when they got ice cream.
He was slaphappiest when he got a new toy.
The puppy was the slaphappiest when it saw its owner.
I was the slaphappiest person in the world when I got my favorite present.
The slaphappiest day of my life was when I went to the amusement park.