The Dictionary of Informal, Colloquial, Slang and Idiomatic Phrases by Hakan Senturk. -A-not know one's arse from one's elbow(v)<slang><impolite> to know nothing.
e.g. Haven't you read his posts on football?! he clearly doesn't know his arse from his elbow.
If you need more examples, put your key words into Google searching box and you will fish more.
Hope this helps.
NB: I am not a teacher.
- For Teachers