It means someone who has achieved a measure of accomplishment in their own field, but area complete blithering idiot outside of this narrow area of expertise.
Who epitomizes this word?
Who could it be?
Oh, I dunno, maybe a world renowned neurosurgeon who makes Donald Trump look smart and sane Presidential Candidate by comparison?
It’s not surprising to find a Surgeon in this position.
One of the open secrets of the medical profession is that doctors work with their minds, and surgeons work with their hands.
Former Senate leader Bill Frist is another classic example of this phenomenon.
Surgery is not a “big picture” line of work. You open you fix, you close.
Diagnosis, by contrast is a process by which you take a constellation of sometimes conflicting information, and create a synthesis.
One of these things involves a big picture and critical thinking.
The other is surgery.