Brewer's: Fallow Land
Land ploughed, but not sown; so called from its brown or tawny colour. (German, fahl, tawny; Anglo-Saxon, falu or fealo, pale-red; hence, fallow deer, red deer.)
“Break up the fallow land.” —Jer. iv. 3.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894