Dictionary.com
Thesaurus.com

cock of the walk

American  

noun

  1. the leader in a group, especially one with a conceited, domineering manner.


cock of the walk British  

noun

  1. informal  a person who asserts himself in a strutting pompous way

"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

cock of the walk Idioms  
  1. A conceited, bossy person, as in Since his last promotion he's been acting like the cock of the walk—he's unbearable. This expression alludes to the rooster's proud strut about the barnyard, asserting his rule over hens and chicks. [Mid-1800s]


Etymology

Origin of cock of the walk

First recorded in 1850–55