tread the boards

From the fact that theatre stages are often made of boards. Compare French monter sur les planches.

tread the boards (third-person singular simple present treads the boards, present participle treading the boards, simple past trod the boards or treaded the boards, past participle trod the boards or trodden the boards)

1. (humorous or dated, figuratively) To work as an theater actor.

   Synonym: walk the boards

   • 1867, Thomas Lake Harris, The Apocalypse:

     The sleek bigot of the conventicle is a more superb actor than the applauded mime who treads the boards

   • 1881, George Bernard Shaw, Love Among the Artists, Book I Chapter VIII:

     This actress was an amiable woman; and Madge enacted Celia in As You Like It at her benefit without any revival of the dread of Shakespeare which the tragedian had implanted in her. She was now beginning to tread the boards with familiar ease. At first, the necessity of falling punctually into prearranged positions on the stage and of making her exits and entrances at prescribed sides, had so preoccupied her that all freedom of attention or identification of herself with the character she represented had been impossible.

   • 1957, Lillian De La Torre, The Actress; Being the Story of Sarah Siddons […] :

     Jack and Stephen, treading the boards together in Dublin, could not be summoned so fast

   • 1999, David Savran, The Playwright's Voice:

     As Mac Wellman's Crowbar so powerfully suggests, every performance, like every theatre building, is haunted by what has come before, by the ghosts of characters and actors who have trod the boards.

   • 2018, Gordon Corera, Operation Columba--The Secret Pigeon Service:

     He spent eight years before the war treading the boards as a professional actor in repertory theaters in Manchester

• “tread the boards” in the Cambridge English Dictionary, Cambridge: Cambridge University Press.
• “tread the boards” in Longman Dictionary of Contemporary English, Longman.
• “tread the boards” (US) / “tread the boards” (UK) in Macmillan English Dictionary.
• “tread the boards”, in Merriam-Webster Online Dictionary, Springfield, Mass.: Merriam-Webster, 1996–present.