noun [C] OLD USE
the man/woman on the Clapham omnibus UK OLD-FASHIONED
an imaginary person whose opinions or ideas are considered to be typical of those of ordinary British people:
The man on the Clapham omnibus probably knows nothing about Rwanda.
The man on the Clapham omnibus is a descriptive formulation of a reasonably educated and intelligent but non-specialist person — a reasonable man, a hypothetical person against whom a defendant's conduct might be judged in an English law
The first reported legal quotation of the phrase is in the case of McQuire v. Western Morning News a libel case, in which Sir Richard Henn Collins MR attributes it to Lord Bowen, who had died nine years earlier.
It is derived from the phrase the bald-headed man at the back of the Clapham omnibus, coined by the 19th century journalist Walter Bagehot to describe the normal man of London, so used because Clapham in south London at the time was a non-descript commuter suburb and was seen to represent "ordinary" London. Omnibus is a now archaic expression for a public bus, but would have been common usage amongst the judiciary at the beginning of the 20th century.