COUSIN (Fr. cousin, Ital. cugino, Late Lat. cosinus, perhaps a popular and familiar abbreviation of consobrinus, which has the same sense in classical Latin), a term of relationship. Children of brothers and sisters are to each other first cousins, or cousins-german; the children of first cousins are to each other second cousins, and so on; the child of a first cousin is to the first cousin of his father or mother a first cousin once removed.
The word cousin has also, since the 16th century, been used by sovereigns as an honorific style in addressing persons of exalted, but not equal sovereign, rank, the term "brother" being reserved as the style used by one sovereign in addressing another. Thus, in Great Britain, dukes, marquesses and earls are addressed by the sovereign in royal writs, etc., as "cousin." In France the kings thus addressed princes of the blood royal, cardinals and archbishops, dukes and peers, the marshals of France, the grand officers of the crown and certain foreign princes. In Spain the right to be thus addressed is a privilege of the grandees.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)