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.
word-ending that sometimes distinguish British from American English. In the U.S., the change from -re to -er (to match pronunciation) in words such as fibre, centre, theatre began in late 18c. and became standard there over the next 25 years at the urging of Noah Webster (the 1804 edition of his speller, and especially his 1806 dictionary). The -re spelling, like -our, however, had the authority of Johnson's dictionary behind it and was unmoved in Britain, where it came to be a point of national pride, contra the Yankees.
Despite Webster's efforts, -re was retained in words with -c- or -g- (such as ogre, acre, the latter of which Webster insisted to the end of his days ought to be aker, and it was so printed in editions of the dictionary during his lifetime). The -re spelling generally is more justified by conservative etymology, based on French antecedents. It is met today in the U.S. only in Theatre as an element in the proper names of entertainment showplaces, where it is perhaps felt to inspire a perception of bon ton.