When you fret, you worry so much about something that it eats away at you. ... If you tell your mother to not fret about you while you're at a sleepover camp, you're telling her to not worry about you too much. Sometimes it means to be agitated though.

What is fret short for?

Förster or fluorescence resonance energy transfer (FRET), resonance energy transfer (RET) or electronic energy transfer (EET) is a mechanism describing energy transfer between two light-sensitive molecules (chromophores).

Do not fret or fret not?

Inf. Don't worry!; Do not fret about it!