The term 'Thieves SLANG' refers to a type of language used by criminals and other lawbreakers to communicate with each other without being understood by outsiders. The two crossword solutions for this term are 'ARGOT' (5 letters) and 'gonofs' (6 letters). ARGOT is a French word that means 'SLANG' or 'CANT, ' and it is often used to refer to the specialized language of criminals. Gonofs is a Yiddish word that means 'thieves' or 'SWINDLERS, ' and it is also used to refer to people who engage in criminal activities.