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Something so FANTASTICALLY stupid or retarded, it ceases to be fantastic and thus becomes tardtastic! Lengthen the 'A' in tard for a more dramatic effect.
"It's a TARDTASTIC day today!"

"This photocopier is acting completely tardtastic today!"

"Wow, that girl in the helmet over there is tardtastic!"

"How ya doing today there, Biff?"
by Zephyr May 04, 2004