1. An abbreviation for "sealed with a kiss," A phrase written on an envelope to indicate that one kissed it to show love for the recipient, perhaps literally and leaving behind a lipstick mark. You must have a secret admirer, because "SWAK" is written on the back of the envelope along with a red lipstick kiss mark.
2. By extension, written and sent with love and affection. This letter comes SWAK, since I can't be there to kiss you in person.
A mark of affection on the back of an envelope. Before e-mail, Facebook, and Twitter, people exchanged romantic sentiments by means of handwritten letters. One way to include a final bit of tenderness was to write S.W.A.K. on the back of the envelope. Women often blotted their lipstick over the four letters to emphasize their love.