1640s, "relating to or characterized by muscular tension," from Greek tonikos "of stretching," from tonos "a stretching," from PIE root *ten- "to stretch." The meaning "maintaining the healthy firmness of tissues" is recorded from 1680s, first extended 1756 to "having the property of restoring to health." Related: Tonical (1580s).
"a tonic medicine," 1799, from tonic (adj.). From 1873 (in gin and tonic) as short for tonic water (1861 as a commercial product, water infused with quinine), so called because held to aid digestion and stimulate appetite.