The most common figure of speech in a situation like the one you describe is to say that something (a conversation, a topic, or especially a joke) is (or goes) over someone’s head, as in sense 1 of this Oxford Dictionaries entry:
Beyond someone’s ability to understand:
the discussion was over my head
If the person who does not understand the conversation is the one saying it, it will often be in a slightly self-deprecating form, such as:
Woah … this is way over my head.
If others say it about him, they are quite likely to come off as fairly condescending.
From the notion of something passing over one’s head come two closely related gestures that are often used together:
- Moving one of your hands quickly over your head (from the front to the back), as if imitating something physically flying past you above your head; and
- Saying, “Whoooosh!”, imitating the sound of someone whooshing past very close to you.
Once again, this is seen as slightly self-deprecating if done by the person who does not understand, and definitely offensive if done by others.
An alternative to expressing that something goes over one’s head is to state that one is out of one’s depth (sense 1.2), an extended sense of a phrase meaning ‘standing in water that is too deep’:
In a situation that is beyond one’s capabilities:
they soon realized they were out of their depth in Division One
I find it difficult to talk in a situation like this—I’m out of my depth
This is a bit more polite and less self-deprecating; it’s also less of an idiom and more of a straightforward collocation. Something like the following is a quite polite way of saying that you don’t understand the topic:
I’m afraid I can’t follow you. I’m a bit out of my depth in this conversation.
is considered beyond reproach
beyond reproach Blameless, faultless, as in Jean's conduct at school is beyond reproach. The phrase employs the verb to reproach in the sense of "censure or rebuke," a usage dating from the early 1500s.
— The American Heritage® Dictionary of Idioms by Christine Ammer
reproach — [...] reprochen "to rebuke, reproach," from Anglo-French repruchier, Old French reprochier "upbraid, blame, accuse, speak ill of," [...]
— etymonline.com
EDIT:
Originally this answer was simply beyond reproach though, as commenters said, this implies subject is in fact perfect, hence added the prefix "is considered" as suggested by @jasper
Best Answer
You are thinking of the man on the Clapham omnibus. In British law he is a hypothetical, reasonably educated, ordinary person you use to compare expected conduct or behaviours with when dealing with things like negligence.
Source: Too much time in the law library while my wife was in law school, and Wikipedia