foe

Etymology 1

From Middle English fo (“foe; hostile”), from earlier ifo (“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 pìktas (“evil”)).

adj

1. (obsolete) Hostile.
   he, I say, could passe into Affrike onely with two simple ships or small barkes, to commit himselfe in a strange and foe countrie, to engage his person, under the power of a barbarous King […]. , vol.1, ch.23

noun

1. An enemy.
   Even without hovering drones, a lurking assassin, a thumping score and a denouement, the real-life story of Edward Snowden, a rogue spy on the run, could be straight out of the cinema. But, as with Hollywood, the subplots and exotic locations may distract from the real message: America’s discomfort and its foes’ glee. 2013-06-29, “Travels and travails”, in The Economist, volume 407, number 8842, page 55

Etymology 2

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.

noun

1. A unit of energy equal to 10⁴⁴ joules.