Cut and pasted from The Answer Bank:
The man on the Clapham Omnibus
Q. Who was he, then
A. The phrase is used in legalese to mean 'the reasonable person', and has gone into the language to mean 'the man in the street', a modern Everyman.
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Q. Everyman
A. Everyman was the central character of the medieval morality play of the same name, which dates from the early 1500s.
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Q. So, the Clapham Omnibus
A. The phrase is said to have been coined by Sir Charles Bowen QC in 1903. While hearing a case for negligence he said (though whether he originated it is not proven, m'lud): 'We must ask ourselves what the man on the Clapham omnibus would think.' Clapham at the turn of the 20th century was a moderately well-off suburb, peopled by 'decent' middle-class types, who, by their staid lifestyle and unchallenging common sense may well have been deemed to be the epitome of ordinariness by the standards of the day, and thus an appropriate yardstick against which the Law Lord might have made a judgement.