On the highest rung' is an idiom that means to be at the top or in the most advantageous position. It is often used to describe someone who has achieved a great deal of success or is in a powerful position. The term can also be used to describe a situation where one is in the best possible position to take advantage of an opportunity. The crossword solution ATOP is a four-letter word that is synonymous with 'on the highest rung'.