From Proto-Indo-European *ménos (“mind”), from *men- (“to think”). Cognates include Avestan 𐬨𐬀𐬥𐬋 (manō) and Sanskrit मनस् (mánas).
μένος • (ménos) (genitive μένεος or μένους); n, third declension;
- desire, ardor, wish, purpose
- courage, spirit, vigor
- power, strength, force
This inflection pattern originally had a sigma (i.e. μένεσος), which was dropped early on in the history of Ancient Greek.
The irregular accentuation of this paradigm is explained by the fact that it is the contracted version of the above paradigm.