Fig.: to be remarkably capable and efficient. To be up to date and alert to new ideas
(slang)

Wow, girls! You did really great! You won the Olympic Gold Medal, proving to the whole world that you're on the ball!

Ragazze, siete state bravissime! Avete vinto la Medaglia d'oro olimpica e avete dimostrato al mondo intero di essere veramente in gamba!

Marisa got promoted to Executive Manager, and she truly deserves it because she's always on top of things and can be trusted beyond doubt

Marisa è stata promossa a Direttore Centrale e se l'è ben meritato perché è sempre sul pezzo e di lei ci si può fidare ciecamente

I recently met Mr. Achilles who, despite his old age, is still very much on the ball: always smart and active, he still runs his business with no outside help

Recentemente ho incontrato il sig. Achille che, nonostante l'età avanzata, è ancora molto in gamba: sempre brillante e attivo, manda avanti ancora il suo business senza aiuti esterni

There's a kid at school who's a real computer wiz. He's only fifteen, but when it comes to computers, he outperforms even his teachers

A scuola c'è un ragazzo in gambissima. Ha solo quindici anni, ma in fatto di computer dà dei punti anche ai professori

This term, from the baseball lingo of early 1900s, relates to the eyes rather than the feet. It is a shortening of the earlier phrase “keep your eye on the ball”, which meant to pay close attention to the speed, spin, or other misleading actions used when throwing a pitch