The story
First put to legal use in a reported judgement by Sir Richard Henn Collins MR in the English Court of Appeal libel case, McQuire v. Western Morning News (1903), who attributed it to Lord Bowen.
Etymology from Wiktionary, CC BY-SA 4.0.
man on the Clapham omnibus
Today it means: Any hypothetical reasonable person used by the Courts in considering questions of reasonableness; an everyman. Full definition →