unbreakablecipher
nounUK /ʌnˈbreɪkəbəl ˈsaɪfə/
US /ʌnˈbreɪkəbəl ˈsaɪfər/
A secret code that nobody can figure out.
The spy wrote her message in an unbreakablecipher so no one could read it.
OTHER MEANINGS
2. A puzzle or riddle that is impossible to solve.
The treasure map's clues formed an unbreakablecipher that stumped every explorer.
3. A very hard problem that seems to have no answer.
Figuring out the bus schedule felt like an unbreakablecipher to the new student.
4. A game or challenge designed so that no player can win.
The final level of the game was an unbreakablecipher that no one had ever beaten.