Imagine a very young boy / girl is going to do something which is hurting your pride and putting you under a question mark. You get seriously annoyed and you want to prevent the child / very young boy/girl from continuing their action. An elder interferes and asks you about the reason behind your annoyance. You tell them:
- A slip of a boy / girl is going to advise me.
Meaning that a not quite mature boy / girl who can be considered as a child is trying to advise me / tell me what to do etc.
Slip of a boy / girl was the only idiom I found for this concept in an old book of English language idioms in our own language. But according to this link I got the fact that it is not much used these days. If so, then please let me know what can I use as an alternative here?