From the French, literally ‘without reproach’. During the 15th century this became a common tag to attach to
knights of very high reputation--in the ideal, a knight should bear a
renown such that no stain could be said to cloud a knight’s character. Because of the idealistic nature of the phrase, I see the phrase as a literal invocation of the ideal image, a useful tool both as a check on reality (see how far from the ideal we really are) and as a way to refocus the activities on the ideal rather than on the gritty reality.
Chivalry always sought perfection, even if it always failed; ‘sans reproach’ was one way of shorthand reference to that ideal.