ABC no A no ji mo nai
one doesn’t even have the character A in ABC.
not have/show the slightest element/sign of ABC; not know the first things about ABC
“ABC” and “A” denote an arbitrary noun or adjective and its initial character, respectively. This is rather a colloquial expression, but when you use it in writing and the initial letter of ABC is kanji, the first hiragana of its reading is often used (see the 2nd example sentence). Note that it usually contains a sarcastic or disapproving tone.
Due to its flexible form, this is probably one of the trickiest Japanese expressions to document in a dictionary (and hence it’s rarely covered). Another example of such an expression is NOUN NOUN した.
He doesn’t know the first thing about fashion.
I helped him with his work but he didn’t show the slightest sign of gratitude.