Sarcastically this adjective means the icing on top of the hypothetical shit cake(a really terrible no good very bad day)
A super obnoxious adjective usually emphasized in a high pitched tone to rub in how extremely wonderful and amazing your day was
To be the best of something that there is. To be great and extremely, utterly wonderful. Not just fantastic, not just fantastical, but fanfuckingtastical. The epitome of fantasticness. It can also be used in a sarcastic manner.
This is the most fanfuckingtastical day of my life!
Sarcastic example: Oh that's just fanfuckingtastical, I'm late to work, haven't eaten breakfast and my tire is flat. Fanfuckingtastical!