"very large diurnal raptorial bird of the genus Aquila," mid-14c., from Old French egle, from Old Provençal aigla, from Latin aquila "black eagle," fem. of aquilus "eagle," often explained as "the dark colored" (bird); see aquiline. The native term was erne.
Golf score sense is by 1908 (according to old golf sources, because it "soars higher" than a birdie). As the name of a U.S. $10 coin minted from 1792 to 1933, established in the 1786 resolution for a new monetary system (but at first only the desperately needed small copper coins were minted). The figurative eagle-eyed "sharp-sighted" (like an eagle) is attested from c. 1600.
c. 1200, "to stretch out, to lay out; diffuse, disseminate" (transitive), also "to advance over a wide area" (intransitive); probably from Old English sprædan "to spread, stretch forth, extend" (especially in tosprædan "to spread out," and gesprædung "spreading"), from Proto-Germanic *spreit- (source also of Danish sprede, Old Swedish spreda, Middle Dutch spreiden, Old High German and German spreiten "to spread"), extended form of PIE root *sper- (4) "to strew" (see sprout (v.)).
Reflexive sense of "to be outspread" is from c. 1300; that of "to extend, expand" is attested from mid-14c. Transitive sense of "make (something) wide" is from late 14c. As an adjective from 1510s. Related: Spreading.
De Vaan writes, "It is possible that 'eagle' was derived from aquilus 'dark' when this had received its colour meaning. It may not be the only dark bird, but it is certainly one of the biggest and most majestic of them." As for aquilus, "The Romans derived this colour from aqua 'water', which [Etymologicum Magnum] reject because they cannot imagine water being black. Still, this seems a more likely derivation to me than from aquila 'eagle' ...."
Meaning "pertaining to an eagle" is from 1650s; that of "eagle-like" is by 1742.