Duncan Bannatyne became especially known for saying “I'm out” after rejecting a pitch on the British Dragon's Den.
The phrase is part of the show's familiar negotiation language. When an investor decides not to proceed, the Dragon formally withdraws from consideration, often using the short declaration “I'm out.” Bannatyne's direct delivery made the expression strongly associated with him in popular coverage of the programme.
Bannatyne was one of the original investors when the British series began in 2005. His business background included nursing homes, health clubs and hotels, and he was known for challenging entrepreneurs about profitability and business fundamentals.
The wording is not a unique legal phrase and other Dragons have used it too. The question identifies the Dragon most popularly associated with the expression, rather than claiming he was the only investor ever to say it.