late 14c., megre (late 12c. as a surname), "lean, thin, emaciated" (of persons or animals), from Old French megre, maigre "thin" (12c.), from Latin macrum (nominative macer) "lean, thin" (source of Spanish, Portuguese, and Italian magro), from PIE root *mak- "long, thin." Compare emaciate.
Of material things (land, food, etc.) from early 15c. Cognate Germanic words (Old Norse magr "thin," Old High German magar, German mager, Middle Dutch magher, Dutch mager, Old English mæger) come directly from the PIE root via Proto-Germanic *magras and are not from Latin.