Fig.: to be alert and watchful in order to avoid being taken off guard; to be ready to act

Everybody in the restaurant staff was on his toes because they knew there was a gourmet expert in incognito

Tutto lo staff del ristorante era sul chi vive perché sapevano che in sala c'era un esperto gourmet in incognito

The Pope will be visiting our city tomorrow, and all the men of the security service are on high alert

Il Papa domani visiterà la nostra città, e gli uomini del servizio di sicurezza sono tutti sul chi va là

In sports like running and boxing. athletes must be physically ready to start or move rapidly, and when they lift their heels and shift their weight to their toes and balls of their feet, they are ready to go!