Take it into your head to do something
Definition 1 of 1
Pro Tip 1/3Impulse Decisions
The phrase often implies acting on an impulse, without much planning or consideration. 
She just took it into her head to buy that expensive dress.
Pro Tip 2/3Sudden Inspiration
It can suggest a stroke of inspiration leading to unplanned action. 
He suddenly took it into his head to paint the kitchen red.
Pro Tip 3/3Unpredictability
Using the phrase can hint at a person's unpredictable nature or behavior. 
Out of nowhere, she took it into her head to adopt a puppy.