Prithee is an old way of saying please. Nobody uses it in ordinary talk now, except in jokes, historical films, or when someone is playing at sounding like Shakespeare. It is a shortening of I pray thee — “I ask you”. In old plays a character might say, “Prithee, sit down.” If you put it in a message today, people will think you are being funny or theatrical, not polite. It belongs with thee, thou, and other words that have left daily English.