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."
"a tonic medicine," 1799, from tonic (adj.).
tonic ton·ic (tŏn'ĭk)
Of or producing tone or tonicity in muscles or tissue.
Characterized by continuous tension or contraction of muscles, as a convulsion or spasm.
Producing or stimulating physical, mental, or emotional vigor.