decoder ring
Noun
Meaning
decoder ring (plural decoder rings)
A device consisting of two concentric rings of letters of the alphabet, one of which can be rotated, so as to encode and decode messages according to a simple substitution cipher.
Examples
Even Charles Darwin, that human decoder ring of bizarre behavior, found the idea of saving a stranger's life to be a total head-scratcher.
A Full Belly: South Indian Restaurant Menu Decoder Ring A weblog about eating well.