The crossword solution for the term 'THICK AS THIEVES, WITH 'IN'' is CAHOOTS. The phrase 'thick as thieves' is an idiom that means two people are very close and trust each other completely. The addition of 'WITH 'in'' implies that the two people are working together in a cooperative manner. Therefore, the word CAHOOTS is an appropriate solution, as it means to be in a close relationship or alliance WITH someone, usually for a common purpose.