c. 1300, from Old English fealh "fallow land," from Proto-Germanic *falgo (source also of Old High German felga "harrow," German Felge "plowed-up fallow land," East Frisian falge "fallow," falgen "to break up ground"), perhaps from a derivation of PIE root *pel- (2) "to fold," hence "to turn." Assimilated since Old English to fallow (adj.), according to OED probably because of the color of plowed earth. Originally "plowed land," then "land plowed but not planted" (1520s). As an adjective, from late 14c.
"pale yellow, brownish yellow," Old English fealu "reddish yellow, yellowish-brown, tawny, dusk-colored" (of flame, birds' feet, a horse, withered grass or leaves, waters, roads), from Proto-Germanic *falwa- (source also of Old Saxon falu, Old Norse fölr, Middle Dutch valu, Dutch vaal, Old High German falo, German falb), from PIE root *pel- (1) "pale." Related: Fallow-deer.