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2.
A person who unfairly, sneakily, or in a general ratty way; gains rewards that they do not deserve!
Nick-The way you stole that girl from me was completely rattastic.

Kap- The way the keeper punched the ball right to your guys head was totally rattastic.
by MiniMuskies September 05, 2011
 
1.
Fantastic.

Originally applied to rat-tails, and how good they look.

Can also be used to describe situations or things that are good.
As applied to hair:
"Hey guys, how does my hair look?"
"Rat-tastic!"

As applied to a situation:
"Wow, that traveling carnival was rat-tastic!"
by Melissa Tasca July 14, 2003