Across the board
Embracing all classes or categories without exception.
The words 'across' and 'board' are of course commonplace in English and so it isn't difficult to imagine them being combined by chance to form 'across the board' in everyday speech and in literature. There are many early examples of the phrase in the context of people talking to each other across a table (a.k.a. board) or when playing board games. Those situations aren't the source of the phrase though - the boards being referred to when the phrase was first used to mean 'encompassing all aspects' were those used by bookmakers to mark up the odds in races. An 'across the board' wager is one in which equal amounts are bet on the same contestant to win, place, or show.
The phrase was coined in America and the earliest example of it that I can find is from The Atlanta Constitution, November 1901:
"Cousin Jess won the steeplechase after a hard drive in the stretch, lowering the best previous time of 4 09 by seven seconds. Dr Einus in the fourth race, a 100 to 1 shot, heavily played across the board, ran second."