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Mr. Big

noun Slang.
  1. a man having the highest authority, control, prestige, or influence in a group, field, situation, or the like, especially in the underworld.

Origin of Mr. Big

First recorded in 1935–40
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for mr big

Mr Big

noun
  1. slang, mainly US the head of an organization, esp of a criminal organization
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012