Late Middle English (in an earlier sense). From Middle French adustion (French (now rare) adustion) state of the humours of being adust, action or process of burning and its etymon classical Latin adustiōn-, adustiō action or process of burning, state of being burnt, (in medical use) burn, heatstroke, sunstroke, in post-classical Latin also cauterization, (of humours) process of becoming adust from adust-, past participial stem of adūrere + -iō.