Where Did the Expression “Think Outside the Box” Come From and What Does the Saying Mean?

The phrase is an allusion to a well-known puzzle where one has to connect nine dots, arranged in a square grid, with four straight lines drawn continuously without pen leaving paper.

The only solution to this puzzle is one where some of the lines extend beyond the border of the grid, or box.

This puzzle was a popular gimmick among management consultants in the 1970s and ’80s as a demonstration of the need to discard unwarranted assumptions, like the assumption that the lines must remain within the grid.