Could anyone help me find a phrase/idiom which is used to describe an ordinary person? I remember it's something like 'a man on ___ bus'. I read it several weeks ago in the Economist or the Guardian, but I can't remember exactly.
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