1
general::
Feeling very happy, delighted, as in She was on top of the world after her roses won first prize. This idiom alludes to the peak of success or happiness. [c. 1920]
American Heritage Idioms
2
general::
Phrase(s): (sitting) on top of the world
Fig. feeling wonderful; glorious; ecstatic. • Wow, I feel on top of the world. • Since he got a new job, he’s on top of the world.
McGrawhill's American Idioms And Phrasal Verbs