According to "Wouldn't say boo to a goose", the idiom's meaning comes from the premise that geese are easily frightened. Assuming, as one commenter stated*, that this is factually false, is there a word or phrase describing this condition?
What is the word that means "In order to understand the meaning of an idiom, you must accept a false statement or premise as true"?
*I have to agree with the commenter. Nobody seems to have told the geese I've encountered of this fact.
Best Answer
Two often misunderstood terms come to mind.
You could say that the idiom begs the question:
People often use "begs the question" to mean "raises the question", but its original meaning was including an assumption in a statement about that assumption- such as the fact that geese are easily scared.
You might also call the idiom a factoid:
People often use "factoid" to mean "a small fact", but its original meaning was a false statement that was repeated so often that people thought it was true- such as the fact that geese are easily scared.