athermancyPhysics.|əˈθɜːmənsɪ| [f.Gr. ἀθέρµαντος not heated: see next.] Athermanous quality; the power of stopping radiant heat. 1863Atkinson Ganot's Physics (1877) §422 Athermancy..corresponds to opacity for light.1870M. Williams Fuel of Sun §111 The quantitative athermancy of flame.