S.W.A.K.

SWAK

1. An acronym 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.
Farlex Dictionary of Idioms.

S.W.A.K.

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.
Endangered Phrases by Steven D. Price
See also:
  • SWALK
  • sealed with a kiss
  • SWAK
  • KISS principle
  • KISS rule
  • kiss (one) on (some place)
  • kiss on
  • T(I)MTOWTDI
  • fish-kiss
  • CAVOK