To join together in force or combine in such a way as to foil or defeat someone or something. The wind and rain conspired against our plans for an outdoor barbecue. His two younger brothers conspired against him to have him removed from the head of the company.

To plot or scheme with one. What are you kids whispering about back there? You better not be conspiring with each other! The president of the company conspired with his assistant to conceal all evidence of his crime.