A shortened version of 'Magnificent Geek'. A play on the word 'Magnifique' from the French 'Magnificent'
A magnifgeek is a geek who is exceptionally competent in their field of expertise, more so than the average geek.
A person who is able to accomplish tasks that seem impossible to the lay person but also difficult to accomplish for the person with common knowledge in the field in question.
That guy is a magnifgeek, he can fix anything motor related.
She's magnifgeek, others in the IT team spent days looking at the problem and she fixed it in 10 minutes!
He's magnifgeek when it comes to sci-fi movies, there is no way he won't know the answer to that question.