From Middle English fo ("foe; hostile"), from earlier ifo, from Old English ġefāh ("enemy"), from fāh, from Proto-West Germanic *faih, from Proto-Germanic *faihaz (compare Old Frisian fāch ("punishable"), Middle High German gevēch ("feuder")), from Proto-Indo-European *peik/k̑- ("to hate, be hostile") (compare Middle Irish óech ("enemy, fiend"), Lithuanian pìktas ("evil")).
Acronym of fifty-one ergs, due to the value of a "foe", 1 foe = 1051ergs; coined by Gerald Brown of Stony Brook University in his work with Hans Bethe.
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