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The emotion evoked when someone says or does something which is not merely 'yay,' but significantly greater 'yay' than normal. Also known as the superlative form of yay.
Girl: "Guess what?"
Guy: "What?"
Girl: *loving kiss* "I love you"
Guy: "Yaytasticness! I love you too."
by xkcdreader August 01, 2010
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