It is (was) + adjective + of somebody + to do something :

This construction is mainly used to show higher intensity in one’s expression, both good and bad. The important point to be noted here is that we use adjective (Not noun) after ‘ It is/  It was ‘ etc

Examples:

• It was foolish of you to accept the proposal . (Not foolishness of you)
• It was very cruel of them to have the dog locked in the cage for days without food and water.( Not cruelty of them)
• Its highly considerate of you to accommodate us.( Not consideration of you)
• It is very kind of you to lend me fifty thousand rupees.
• It was bad of him to behave like that.
• It is intelligent of you to answer this question.
• It is very kind of you to do the kitchen chores for the old lady.