S.W.A.K.

sealed with a kiss (written at the end of a love letter or on the back of its envelope).

Also SWAK

[swak] /swæk/.

Origin of S.W.A.K.

First recorded in 1925–30

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for swak

“SWAK” is where we see that Tony is more than a wise ass in a suit.
NCIS’s 11th Anniversary: Michael Weatherly’s Top 10 Moments|Michael Weatherly|September 23, 2013|DAILY BEAST
“SWAK,” or sealed with a kiss, is the virus episode where I get the plague.
NCIS’s 11th Anniversary: Michael Weatherly’s Top 10 Moments|Michael Weatherly|September 23, 2013|DAILY BEAST

Word Origin and History for swak

SWAK

acronym for sealed with a kiss, attested from 1911, in a legal publication quoting a letter from 1909:

"... Well Kid I don't know nothing else to say only that I hope to see your sweet face Sat. Good by from your Dear Husban to his sweet little wife. P. S. excuse bad writing and mispelled words take all mistakes as kisses. S.W.A.K. * * *" This letter was postmarked at Des Moines October 20, 1909, addressed to Carrie Sprague at Jefferson, Iowa, and reached the latter place October 21, 1909. [State v. Manning (a conspiracy to lure women into prostitution case), Supreme Court of Iowa, Nov. 16, 1910, reported in "Northwestern Reporter," Volume 128, 1911]

Popularized in soldiers' letters home in World War I.