diet:  Diet comes, via Old French diete and Latin diaeta, from Greek díaita ‘mode of life’. This was used by medical writers, such as Hippocrates, in the specific sense ‘prescribed mode of life’, and hence ‘prescribed regimen of food’. It has been speculated that Latin diaeta, presumably in the yet further restricted sense ‘day’s allowance of food’, came to be associated with Latin diēs ‘day’.
This gave rise to medieval Latin diēta ‘day’s journey’, ‘day’s work’, etc, hence ‘day appointed for a meeting’, and thus ‘meeting (of legislators)’. English acquired this word (coming orthographically full circle as diet) in the 15th century, but it is now mainly used for referring to various foreign legislatures.
"regular food," early 13c., from Old French diete (13c.) "diet, pittance, fare," from Medieval Latin dieta "parliamentary assembly," also "a day's work, diet, daily food allowance," 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" + ainysthai "take," from PIE root *ai- (1) "to give, allot." Often with a sense of restriction since 14c.; hence put (someone) on a diet (mid-15c.).
"assembly," mid-15c., from Medieval Latin dieta, variant of diaeta "daily office (of the Church), daily duty, assembly, meeting of counselors," from Greek diaita (see diet (n.1)), but associated with Latin dies "day" (see diurnal).
late 14c., "to regulate one's diet for the sake of health," from Old French dieter, from diete (see diet (n.1)); meaning "to regulate oneself as to food" (especially against fatness) is from 1650s. Related: Dieted; dieting. An obsolete word for this is banting. The adjective in this sense (Diet Coke, etc.) is from 1963, originally American English.