The man on the Clapham omnibus
The man on the Clapham omnibus is a hypothetical reasonable person, used by the courts in English law where it is necessary to decide whether a party has acted in the way that a reasonable person should - for example, in a civil action for negligence. The man on the Clapham omnibus is a reasonably educated and intelligent but nondescript person, against whom the conduct of the defendant can be measured. This concept was used by Greer LJ in the case of Hall v. Brooklands Auto-Racing Club (1933) to define the standard of care which a defendant must live up to in order to avoid being found negligent.
The phrase was first put to legal use in a reported judgment by Sir Richard Henn Collins MR in the 1903 English Court of Appeal libel case, McQuire v. Western Morning News. He attributed it to Lord Bowen, said to have coined it as junior counsel defending the Tichborne Claimant case in 1871. Brewer's also lists this as a possible first use.
It is possibly derived from the phrase "Public opinion ... is the opinion of the bald-headed man at the back of the omnibus," coined by the 19th century journalist Walter Bagehot to describe the normal man of London. Clapham in south London at the time was a nondescript commuter suburb seen to represent "ordinary" London. Omnibus is now rather an archaic expression for a public bus, but was in common use by the judiciary at the beginning of the 20th century.
See also 
-  2 KB 100 (CA) at 109 per Collins MR
- Room, Richard, ed. (1996). Brewer's Dictionary of Phrase and Fable (15th ed.). Cassell. p. 761. ISBN 0062701339.
- Bagehot, Walter (1867, 1894). "The English Constitution".
- Richard, Hughes G. (1989). "The Beloit-Valmet saga continues". Leger Robic Richard / Robic. Retrieved 14 July 2010.
- Asprey, Michèle M. (2003, revised 2010). Plain Language for Lawyers. Federation Press. p. 119. ISBN 978-1-86287-775-7.
- "Re Sortirios Pandos and Commonwealth of Australia  AATA 18 (25 January 1991)".
- "Ng Chiu Mui v Securities and Futures Commission Application No 7 of 2007 (15 May 2009".