It’s rhyming slang with ‘nix’ which is Latin, and means to nullify or cancel. Because there layers of meaning hidden in english, Latin, and arabic numbers is not possible to be confusing.
And not to be confused with ‘deep 6’ which means to destroy, kill orr bury something 6 feet deep.
Instructing kitchen to deep 6 the cherries, the line cooks gonna need a gun and a shovel.