"external behavior (especially polite behavior) in social intercourse," late 14c., plural of manner in a specific sense of "proper behavior, commendable habits of conduct" (c. 1300).
Under bad manners, as under graver faults, lies very commonly an overestimate of our special individuality, as distinguished from our generic humanity. [Oliver W. Holmes, "The Professor at the Breakfast Table," 1858]
Earlier it meant "moral character" (early 13c.).
MANNERS-BIT, a portion of a dish left by the guests that the host may not feel himself reproached for insufficient preparation. [Rev. Joseph Hunter, "The Hallamshire Glossary," 1829]