Both a physically and mentally-retarded person. Their face is usually deformed too.
Did you see Kyle from Cardiff High School. He has a walking cane, his eyes and right leg are deformed and he might be retarded too. He's retastic.
Basically another way of saying retarded but when retarded doesnt cut it theres retastic which is worse than retarded. Can be used literally or not.A mix of retarded and the end of fantastic but using it to describe something they are definatly not fantastic at all. Usually used to describe someone who is severly phyiscally handicapped and over-super severly mentally retarded, there face is usually deformed too.
Literal sense: Person 1: Dude did you see that kid that was eating his own snot,drooling with his mouth open and poking his mole at the same time.
Person 2: Yeah that kids retastic
Non-literal sense: Person 1; AHAHAHAHA IM A LEET HACKER YOU SUCK YOUR A PRICK YOU DICK LLEET HACKS FOR THE WIN.
Person 2: Your fucking retastic.