A courtesan was originally a female courtier, which means a person who attends the court of a monarch or other powerful person.
In feudal society, the court was the centre of government as well as the residence of the monarch, and social and political life were often completely mixed together. Prior to the Renaissance, courtesans served to convey information untrusted to servants to visiting dignitaries. In Renaissance Europe, courtiers played an extremely important role in upper-class society. As it was customary during this time for royal couples to lead separate lives — commonly marrying simply to preserve bloodlines and to secure political alliances — men and women would often seek gratification and companionship from people living at court. In fact, the verb “to court” originally meant “to be or reside at court”, and later came to mean “to behave as a courtier” and then “to pay amorous attention to somebody”. The most intimate companion of a ruler was called the favourite.
In Renaissance usage, the Italian word “cortigiana”, feminine of “cortigiano” (courtier) came to refer to “the ruler’s mistress”, and then to a well-educated and independent woman of free morals, eventually a trained artisan of dance and singing, especially one associated with wealthy, powerful, or upper-class men who provided luxuries and status in exchange for companionship. The word was borrowed by English from Italian through the French form “courtisane” during the 16th century, especially associated to the meaning of “court-mistress” and “prostitute”.
A male figure comparable to the courtesan was the Italian cicisbeo, the French chevalier servant, the Spanish cortejo or estrecho. It actually seems that the figure of the chevalier servant (French, lit. serving cavalier, lady’s escort) of a married lady was quite common in Europe up to the 18th century.
Today, the term courtesan has become a euphemism to designate an escort or a prostitute, especially one who attracts wealthy clients.