The officious bystander is a metaphorical figure of English law and legal fiction, developed by MacKinnon LJ in Southern Foundries (1926) Ltd v Shirlaw[1] to assist in determining when a term should be implied into an agreement.
The suggested approach is to imagine a nosey, officious bystander walking past two contracting parties and asking them whether they would want to put some express term into the agreement.
The rule now is that terms are implied to reflect the parties' reasonable expectations as a broader part of the process of objective, contextual construction.
The imaginary conversation with an officious bystander in Shirlaw v Southern Foundries (1926) Ltd [1939] 2 KB 206, 227 is celebrated throughout the common law world.
Like the phrase "necessary to give business efficacy", it vividly emphasises the need for the court to be satisfied that the proposed implication spells out what the contract would reasonably be understood to mean.