tonic
n.
1
"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.
2
in the musical sense, 1760, short for tonic note, from tone, n. in the musical sense + -ic. Related: Tonicity.
adj.
1640s, "relating to or characterized by muscular tension," from Greek tonikos "of stretching," from tonos "a stretching" (see tenet). 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).