1756, "having to do with hot springs," from French thermal (Buffon), from Greek therme "heat, feverish heat," from PIE *gwher- "to heat, warm" (cognates: Latin fornax "an oven, kiln," formus "warm," Old English wearm; see warm, adj.). Sense of "having to do with heat" is first recorded 1837. The noun meaning "rising current of relatively warm air" is recorded from 1933.