Green’s Dictionary of Slang

sealed with a loving kiss; usu. found on the back of envelopes; other versions include s.w.a.k., sealed with a kiss; s.w.a.n.k., sealed with a nice kiss; and s.w.a.l.c.a.k.w.s., sealed with a lick ’cos a kiss won’t stick.

1915		P.W. Long ‘Semi-Secret Abbreviations’ in DN IV:iii 246: c.y.k. Consider yourself kissed: – epistolary endearment. Also s.w.a.k. (sealed with a kiss).
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 274: SWAK: [...] Sealed with a Kiss. A common superscription on the envelopes of letters to sweethearts from sailors and soldiers. Sometimes also S.W.A.N.K.—i.e., Sealed With A Nice Kiss.
1939		A. Kober Pardon Me for Pointing 176: Yeah, ‘SWAK.’ You know, it’s initials meaning, ‘Sealed with a kiss.’.
1945		D. Bolster Roll On My Twelve 90: What letters! [...] always covered in kisses and hearts and SWALK.
1968		‘Hy Lit’ Hy Lit’s Unbelievable Dict. of Hip Words 52: swak – Sealed with a kiss.
1977		(con. 1949) J.G. Dunne True Confessions (1979) 161: She’d draw the little heart with the arrow through it, and inside the heart it would say ‘Mary and Des.’ And ‘S.W.A.K.’ She wrote that, too.
1979		H. Leonard A Life (1981) Act II: desmond: Why’d your father open the letter? mibs: Because I never get any. ’Specially ones with ‘S.W.A.L.K.’ on one side and ‘S.A.G.’ on the other.
1983		J. Ciardi A Second Browser’s Dict. 261: SWAK. Sealed With A Kiss.
1990		R. Doyle Snapper 112: She could make out the shapes of the rubbed-out writing on the flap. They were capital letters. S.W.A.L.K.
1996		(con. 1960s) D. Ó Muirithe Words We Use 82: Of course boarding school wide-boys got in on the act; many of my generation will have seen things like SWALCAKWS (sealed with a lick ’cause a kiss won’t stick).
2002		‘Hectic James’ HecticJames.com ‘Stuff’ 30 Sept. 🌐 These are shorthand acronyms used by lovers, such as SWALK for Sealed with a loving kiss. Other examples given were: BURMA: Be undressed ready, my angel; EGYPT: Eager to grab your pretty tits; HOLLAND: Hope our love lasts and never dies; ITALY: I trust and love you; NORWICH: (K)nickers off ready when I come home; BOLTOP: Better on lips than on paper (of a kiss).
2003		B. Coultas Handmade Museum 53: Signed SWALCAKWS (sealed with a lick cuz a kiss won’t stick), Brenda.