Green’s Dictionary of Slang

merry widow n.²

(US) a condom.

1929		E. Dahlberg Bottom Dogs 263: He took to carrying merrywidows in his vestpocket.
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 454: Mickey Flannagan slept in the corner [...] Barney Keefe folded his hands, and placed a soggy Merry Widow in them.
1935		J. Conroy World to Win 194: Three for a quarter, and genuine Merry Widows. Won’t rip, ravel, tear or run down at the heels.
1992		(con. 1920s) F.M. Davis Livin’ the Blues 37: To avoid ‘blue balls,’ a dose, or sores with a strange broad you used either a Fish Skin or a thin rubber called a Merry Widow.