be (sitting) on top of the world

1. To be elated; to be blissfully happy. I was on top of the world when I found out that I'd gotten an A on my hardest exam. Cathy has been sitting on top of the world ever since she got that big promotion at work. I know we've been engaged for a whole week now, but I'm still sitting on top of the world!

2. To be extremely successful; to have everything one wants or desires. She owns three of the world's largest companies and is one of the richest people on the planet. She's on top of the world right now. Even though he was sitting on top of the world after winning his sixth Olympic gold medal, he still craved more in life. Now that they've sold their tech start-up for a billion dollars, those guys are definitely sitting on top on of the world.

on top of the world, to be

To feel elated; to be extremely successful. This metaphor for the peak of well-being dates from the early twentieth century. P. G. Wodehouse used it in Very Good, Jeeves! (1930): “If ever a bird [fellow] was sitting on top of the world, that bird was Bingo.”