▻ FALLACY

I take this broadly as a logical mistake.

▻ NO FALLACY - REASONABLE STATEMENT

There are uses of 'You are an X, therefore you should know Y' which are perfectly reasonably. If I approached a Harvard professor of math., asked 'what is a prime number?', and they professed total ignorance, 'You are a mathematician, therefore you should know' would be completely in order in regard to so basic and elementary a matter.

▻ NO FALLACY BUT UNREASONABLE EXPECTATION

By contrast if I asked a math teacher who teaches only basic math. what Betti numbers are, this is a relatively advanced matter and it would not be reasonable to say 'You are a mathematician, therefore you should know'. The nature of Betti numbers is not something that just any mathematician can be reasonably expected to know.

▻ WHEN TO USE 'YOU ARE AN X, THEREFORE YOU SHOULD KNOW Y'

In sum 'You are an X, therefore you should know Y' is reasonable only if Y is something that falls within the standard knowledge of any and every X.

▻ THE OPEN-TEXTURE OF STANDARD KNOWLEDGE

Now the question gets interesting since it is often contestable what does fall within standard knowledge. There is often no sharp line to be drawn at which to decide where 'standard knowledge' ends - the concept is open-textured. In your linguistic example I would expect any French graduate to know the French for a 'shop' or a 'street'. But the French for 'a 'wheelbarrow' ? It's on or near the edge of everyday speech. One could spend years in France without needing to use the word (*brouette*, I think). As not definitely calling on standard knowledge, 'you should know' is an unreasonable expectation in the wheelbarrow case.

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The title does not match the body of the post. The example in the body describes appeal to non-authority. Its use in the last paragraph is a tactic not at all specific to this particular fallacy, and is known as poisoning the well.

– Conifold – 2017-10-26T00:04:28.810Not sure that in general terms one can talk of a fallacy here. 'You are a mathematician, therefore you ought know what a logarithm is' or 'You are a plumber, therefore you ought to know why my sink is blocked' seem well-grounded 'therefores'. In these cases if S claims to be an expert, then there are certain things which S can reasonably be expected to know - 'ought to know' in one sense of the phrase. Where 'therefore' is unfairly used is in a case such as this : S is and claims to be a historian. – Geoffrey Thomas – 2017-12-22T19:40:51.707

BTW, Fallacy is "weak logical reasoning". It's not the same thing as logical negative, i.e. false. – mavavilj – 2018-05-22T12:25:08.010