A tonic is something, sometimes a medicine, that makes you feel better or restores you to health. Before doctors prescribed drugs, crooked salesmen sold all sorts of dubious tonics.
Tonic was used to describe all sorts of concoctions that were supposed to make you feel better or livelier. Some helped and some didn’t. Coca Cola was originally marketed as a tonic — back when it is said to have contained cocaine as well as loads of caffeine. These days we might describe a weekend at the beach or a brisk walk as a tonic, meaning it will refresh you. Others might flee to a gin and tonic — an alcoholic drink.