Green’s Dictionary of Slang

1. a letter.

1905

Duke Tritton’s Letter n.p.: Well China, this Don’t Forget Her is getting long and I am out of Nails And Screws, and I have to catch the Holy Ghost, so will end off with all the best from, Your old Thief And Robber.

2. a condom, a ‘French letter’.

1984

J. Meredith Learn to talk the Old Jack Lang 23: don’t forget her: a letter, a french letter.