man on the Clapham omnibus

Etymology
First put to legal use in a reported judgement by Sir Richard Henn Collins MR in the 1903 English Court of Appeal libel case, McQuire v. Western Morning News, who attributed it to Lord Bowen.

Noun

 * 1)  Any hypothetical reasonable person used by the Courts in considering questions of reasonableness; an everyman.
 * 2)  Any ordinary person; everyman.