A shackle code is a cryptographic system used in radio communications on the battle field by the US military, the Rhodesian Army, and the Canadian Army, among other English speaking militaries which might not distribute or require sophisticated one-time use pads.
Each of the letters of the English alphabet were assigned a numeric value.
The assignation was changed frequently and required the distribution of the codes to each party in advance.
When a party wanted to communicate a number, it radioed "SHACKLE" and it spelled out each digit (or combination of digits) using a word starting with the letter.
During World War II, the shackle codes took time to encode and decode, so during battle, sometimes troops radioed English plaintext with profanity.