c. 1200, "regular food," from Old French diete (13c.) "diet, pittance, fare," from Medieval Latin dieta "parliamentary assembly," also "a day's work; daily food allowance, food," from Latin diaeta "prescribed way of life," from Greek diaita, originally "way of life, regimen, dwelling," related to diaitasthai "lead one's life," and from diaitan, originally "separate, select" (food and drink), frequentative of *diainysthai "take apart," from dia "apart" (see dia-) + ainysthai "take," from PIE root *ai- (1) "to give, allot."
From late 14c. as "customary way of eating," also "food considered in relation to its quantity and effects," and "a course of food regulated by a physician or by medical rules," often a restriction of food or certain foods; hence to put (someone) on a diet (mid-15c.). The adjective in the sense of "slimming, having reduced calories" (Diet Coke, etc.) is attested by 1963, originally in American English.
"assembly of delegates, etc., held from day to day for legislative, political, or other business," mid-15c., from Medieval Latin dieta, variant of diaeta "daily office (of the Church), daily duty, assembly, meeting of counselors," from Greek diaita "regimen" (see diet (n.1)), but associated with Latin dies "day" (from PIE root *dyeu- "to shine"). Since c. 1600 used by English and French writers of the legislative assemblies of Germany and Austria.