Origin and history of etiology

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). This apparently is a frequentative of *diainysthai "take apart," from dia "apart" (see dia-) + ainysthai "take," which according to Watkins is 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. 

word-forming element meaning "a speaking, discourse, treatise, doctrine, theory, science," from Medieval Latin -logia, French -logie, and directly from Greek -logia, from -log-, combining form of legein "to speak, tell;" thus, "the character or deportment of one who speaks or treats of (a certain subject);" from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')."

In philology "love of learning; love of words or discourse," apology, doxology, analogy, trilogy, eulogy, etc., Greek logos "word, speech, statement, discourse" is directly concerned.

word-forming element used in chemistry and indicating "a fundamental degradation product of a complex organic compound" [Flood], from Latinized combining form of Greek aitia "a cause, an origin" (see etiology). In older, general use it has been reduced in English to etio- (see æ (1)).