Butter is a bread-and-butter vocabulary word, but it may have spread all the way from ancient Scythia.
Atkins, Paleo, Flexitarian? If you want to find a diet that works, try etymology.
The word diet* is no trend. The Oxford English Dictionary attests the word as early as 1230 in a monastic manual. Even then, the term referred to food, particularly those victuals “in daily use” and “in relation to their quality and effects” (OED). But in 1460, the word did enlarge its meaning to include “course of life,” yielding expressions such as of the same diet, of a different diet, and both of a diet, the OED tells us. And this “course of life” points us right to its etymology.
English, as we know, maintains a hearty Mediterranean diet while also eating generously of French cuisine. Diet comes from the Old French diète, “food” and “fare,” in turn from the Latin diæta, a “daily allowance,” “ration of food,” or “mode of life.” The latter sense shows that Latin put Greek feta on everything: diæta is directly from the Greek δίαιτα (diaita), which Liddell and Scott gloss as a “way of living” or (here it is again) “mode of life.” The noun could also signify “dwelling,” “abode,” and “room,” as well as “arbitration.”
Liddell and Scott connect δίαιτα to verbs meaning “to lead a certain course of life.” Others narrow it down to core verbs like “maintain” or “separate.” At core may be ζάω (zao, ultimately connected to zoo, I believe), which means “to live.” Eric Partridge and Ernest Klein, though, break δίαιτα apart into δία– (“through,” “apart”) and αἰσα, “dispensation of a god,” hence “one’s appointed lot” or “destiny.” Now, that’s a heavy diet. The first element of etiology is believed to be related, via the Greek αἰτία (“charge,” “guilt”).
For some Proto-Indo-Europeanists, the ultimate root is *ai–, “to give” or “allot” (American Heritage Dictionary of Indo-European Roots). So, if you are dieting, don’t just pass over those holiday chocolates still lingering around the office. Also pass over all the clickbait advertising top diet tricks and trends. For, the etymology of diet may well give you the best advice: It’s all what you allot yourself, what course you follow, what lifestyle you live. Now, if I could only do something about that media diet of mine…
* Diet, as in an “assembly” like the Diet of Worms, could be from the same root as our focal word here but influenced by the Latin dies, “day.”