Fantabulous is an adjective meaning something is of the highest quality. It is a combination of fantastic and fabulous, which was commonly used in London in the 1960s and 70s to describe various aspects of pop culture and fashions. It was especially popular with Chelsea girls like Joanna Lumley. It is still in use in England today, but not as much as it used to be.
That new coat of yours looks absolutely fantabulous!