foe

From Middle English fo (“foe; hostile”), from earlier yfoh, yvo, ifa (“foe”), from Old English ġefāh (“enemy”), from fāh (“hostile”), from Proto-West Germanic *faih, from Proto-Germanic *faihaz (compare Old Frisian fāch (“punishable”), Middle High German gevēch (“feuder”)), from Proto-Indo-European *peyk/ḱ- (“to hate, be hostile”) (compare Middle Irish óech (“enemy, fiend”), Lithuanian pi̇̀ktas (“evil”)).

Acronym of [ten to the power of] fifty-one ergs, due to equalling 1051 ergs; coined by Gerald Brown of Stony Brook University in his work with Hans Bethe.