The term 'DROP IN A LETTERBOX' can be broken down into two words, 'MAIL' and 'DROP'. The term 'MAIL' is a four letter word which refers to the act of sending letters, packages, or other items through the postal service. The term 'DROP' is a verb which means to put something in a place quickly and without much effort. Therefore, the phrase 'DROP IN A LETTERBOX' refers to the act of putting MAIL into a mailbox for delivery by the postal service.