I'm trying to understand how offensive the word Fachidiot is.
It can be very offensive.
The term (which could be literally translated as "Expertise Idiot") implies that the person is over-specialized in their expertise to the point of being dysfunctional. Not just dysfunctional at unrelated tasks but even at those tasks they are supposed to be experts in due to the inability to view a problem from different perspectives and their lack of soft skills. It also implies that the person is too arrogant to realize this.
So when you call someone a "Fachidiot" during an argument, you essentially say "You are so stuck up in your expertise that you fail to perceive reality and therefor it is pointless to debate with you". The insulting thing about this is that you attack what they perceive as their greatest strength and claim that it is in fact a weakness.
Further reading: German Wikipedia
But as with any offensive term: Context matters. Any term can be insulting or harmless, depending on how you say it, in what situation you say it and what relationship you have with the person you are saying it to.
Nein, ja, ja, kommt drauf an.
Über den Betreffenden wird gesagt, dass er zwar Experte auf seinem Gebiet ist, aber ansonsten Scheuklappen aufhat. Ein Kompliment ist das nicht. Wenn man aber weiß, dass der Betreffende sich selbst so sieht und den Zustand frei gewählt hat, etwa weil er alles außer seinem Fachgebiet vernachlässigt, um in diesem an der Spitze zu sein, dann kann man es sagen, oder wenn es nicht so ernst gemeint ist, etwa wenn ein Mitarbeiter meint die Grafikkarte sei kaputt, und man sagt "Holger ist ein Fachidiot - er kann nur Software", während man den Powerknopf des Bildschirms drückt, um ihn einzuschalten.
Wie so oft kommt es darauf an, wie der Kontext der Situation ist, wie man es meint, wie man es ausdrückt (Stimme, Gestik, Mimik), wie empfindlich oder entspannt, selbstkritisch und humorvoll der Gemeinte ist und ob er es so versteht, wie es gemeint ist.
Ist es aber als ernstgemeinte Kritik geäußert, dann mildert die Anerkennung der fachlichen Kompetenz diese immerhin ein wenig ab.
translation by B--rian
No, yeah, yeah, it depends.
It is said about the person concerned that, although he is an expert in his field, he otherwise has blinders on. That is not a compliment. But if you know that the person concerned sees himself that way and has chosen the state freely, for example because he neglects everything except his field of expertise in order to be at the top in this one, then you can say it, or if it is not meant so seriously, for example if an employee thinks the graphics card is broken, and you say "Holger is a professional idiot - he can only do software" while you press the power button on the screen to turn it on.
As so often, it depends on how the context of the situation is, how you think it is, how you express it (voice, gestures, facial expressions), how sensitive or relaxed, self-critical and humorous it is and whether you understand it as it is meant.
If, however, it is expressed as serious criticism, then the recognition of professional competence at least softens it a little.
From my point of view:
To sum up, I feel that the term does have a high risk of offending someone and would therefore never use it in professional communication.
There exists only one setting where you can use the term "Fachidiot" outside of bantering with (rather good) friends: when you are referring to yourself (hopefully in jest).
Every other usage will be considered rude.
This may include the recognition of any given expertise, but it is rather directed at the limited scope someone may have (good at one thing, useless for everything else). In general, it is the exact opposite of "Jack of all trades" (which implies a wide scope in expertise, even if the detail-knowledge may be shallow). "Trained monkey" might be the English term which comes the most close, because indoctrination is no education - and indoctrination produces this kind of people.
In a professional setting, this term should better be avoided, because it can be interpreted as an insult, even if it may describe a person accurately. And when the term is used, this often happens among coworkers of that person, who are fed up of issues arising from that limited scope. Especially, when the problems which arise are outside of the limited scope of the person who caused them. It's usually far more an expression of anger, than it would be useful to insult somebody.
Karl Marx put it nicely:
Was die Arbeitsteilung in der modernen Gesellschaft charakterisiert, ist die Tatsache, dass sie die Spezialitäten, die Fachleute und mit ihnen den Fachidiotismus erzeugt.
In multi-disciplinary teams, these are the people who are barely interested in the other disciplines.
Derived from "Fachmann" (specialist), as its tongue-in-cheek modification, or escalation. Its insult level is a bit lower than that of pure idiot, as the person is acknowledged as a legitimate specialist, at least.
Contrary to the other answers, I would argue that a Fachidiot is an idiot in the discipline. As such it would have been wrong to write "their own discpline" in the previous sentence. It is pretty much impossible to become an expert at anything, and own an entire field (whether alone or in a group), if a broadly diversified basis is missing.
The difference is fleeting, of course, because the concept of expertiese can be expressed variously and especially with irony when it's supposed to display the inverse magnitude of idocy. It's bounded, because opposite to no expertise is just expertise with an unspecified magnitude. The expertise can be expressed relative in quality or quantity to the common sense. Still, the idocy can aproach infinity, either way. And so there's an almost unlimited potential for being and being taken as offensive. The word idiot is always offensive, no matter how it is packaged up.
But the irony can be taken a step further, if the general common knowledge is not really doubted, but pales in comparison to the specific knowledge. Then you just have somebody vom Fach, a Fachmann, Facharbeiter, or Bereichsleiter, who is an idiot for other reasons, perhaps because they downvote answers without commenting. The joke is anyway that it's an oxymoron, and so it's not funny anymore if the ambiguity is missing. Also cp fähig, ein fähiger Idiot.
It could at least express uncertainty, however.
Now the question whether something offensive is always offensive is interesting, but the answer is simply:
No, you idiot!
