(v.) to worry
When you fret about something, you’re literally ‘eaten’ away by it.
Fret originally meant to eat ravenously when it first emerged in our language in the Old English period. Through English’s Germanic roots, it’s a none-too distant cousin of the German word fressen—the animal equivalent of essen, meaning ‘eat’.
German is actually one of a number of languages today that recognise a difference between people and animals eating. So while essen (related to eat) is used of people, fressen (related to fret) is used of animals.
Fret hasn’t much survived in this sense in English, though you might occasionally encounter it in a much lighter sense meaning simply to gnaw, or wear away at something; woodworms, for instance, are still sometimes said to fret timber. But in its original sense of devouring or consuming like a wild beast, fret has long since fallen out of use.
It has survived as a verb in a more figurative sense, however: to fret now means to worry. Admittedly, it’s been suggested here that there may be some confusion with the French verb frotter, meaning to rub. But this striking change in meaning was just as likely driven by the figurative notion that concerns can ‘eat away’ and undermine a person, just as woodworms sap the strength from beams and timbers.