OED “the ordinary or average man; the ‘man in the street’.”
1903 Law Rep. (King’s Bench Div.) II. 109 ‘Fair’, therefore, in this collocation certainly does not mean that which the ordinary reasonable man, ‘the man on the Clapham omnibus’, as Lord Bowen phrased it, the juryman common or special, would think a correct appreciation of the work.
Just out of interest, is there an American equivalent?