(… وَمَكَرُوا) (conspired …): This Arabic word originally refers to the trees whose trunks and branches are small and wrapped together that you cannot distinguish each one. Likewise, to conspire means to scheme against your opponent secretly. A person who prevaricates is the one who conspires. If conspiring is not meant for harm, then it is called a trick, but if it is for the purpose of harm, then it is called the bad conspiracy which based on premeditation.

(  مَكْرًا كُبَّارًا… ) (…an immense conspiracy): Allah (swt) describes this conspiracy as being an immense, meaning great and  extraordinary because the people of Noah (PBUH) conspired to put an end to his Call.