Meaning of the word "not know B from a battledore" in English

What does "not know B from a battledore" mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

not know B from a battledore

US /nɑːt noʊ biː frʌm ə ˈbæt.əl.dɔːr/
UK /nɒt nəʊ biː frəm ə ˈbæt.əl.dɔːr/
"not know B from a battledore" picture

Idiom

to be completely ignorant or illiterate; to not know even the simplest things

Example:
He claims to be an expert, but he does not know B from a battledore.
How can she manage the project when she does not know B from a battledore about finance?