A pen pal greeting is a message sent between two people who have never met in person, but communicate through letters or emails. The term 'OINK' is a four-letter word that can be used as a pen pal greeting. It is an acronym for 'Only In Name Known, ' which is a way of saying that the sender does not know the recipient's name. OINK is often used as a friendly and informal way to start a conversation with a new pen pal.