argumentum ad dictionarium 

Using a language dictionary to argue your points, to justify your actions, and/or to claim that all uses of a given word outside of a given dictionary are incorrect. Fallacious as English dictionaries do not prescribe usage, but merely describe how words are used by society.

Person 1: Stop treating your 35 year old son like a child. He's not a child anymore.
Person 2: But the dictionary says so. *pulls out Oxford* A child is "a son or daughter of any age". My 35 year old is my son, therefore he is a child. That means my 35 year old is not ready to watch anything but G rated movies, cannot drink beer, must obey my commands at all times, isn't old enough for a job, and...
Person 1: Classic argumentum ad dictionarium.

argumentum ad dictionarium by udusers1 August 18, 2016