1911 Encyclopædia Britannica/Courtesy
COURTESY (O. Fr. curtesie, later courtoisie), manners or behaviour that suit a court, politeness, due consideration for others. A special application of the word is in the expression “by courtesy,” where something is granted out of favour and not of right, hence “courtesy” titles, i.e. those titles of rank which are given by custom to the eldest sons of dukes, marquesses and earls, usually the second title held by the father; to the younger sons and to the daughters of dukes and marquesses, viz. the prefix “lord” and “lady” with the Christian and surname. For “tenure by the courtesy” see Curtesy. Another form of the word, “curtsey” or “curtsy,” was early confined to the expression of courtesy or respect by a gesture or bow, now only of the reverence made by a woman, consisting in a bending of the knees accompanied by a lowering of the body.