Noun. A derogatory term for a one-track specialist who is an expert in his field, but takes a blinkered approach to multi-faceted problems.
The word originates from the German but there is no suitable translation into English. A “one-track specialist” is not quite right because you would not call someone like that an “idiot”. The “fach” comes from the German for “subject”. Example: “Despite being an expert in horticulture, the manager came across as something of a fachidiot when dealing with translation issues.”