Someone who is so amazing and incredible that no known adjective can describe them. This word is used when you are so happy with someone, that you can't even think of a word to call them.
Man, my girlfriend Abigail is soooo magnifantastic! she just makes me so happy all the time, and is just such an amazing person.
Abigail you're....whats the word? MAGNIFANTASTIC!! I can't believe how awesome you are!