Open and sincere in expression; straightforward: made several frank remarks about the quality of their work. Clearly manifest; evident: frank enjoyment. To put an official mark on (a piece of mail) so that it can be sent free of charge. To send (mail) free of charge. To place a stamp or mark on (a piece of mail) to show the payment of postage. To enable (a person) to come and go freely. A mark or signature placed on a piece of mail to indicate the right to send it free of charge. The right to send mail free. A franked piece of mail. Informal A frankfurter.