Be off your rocker
Definition 1 of 1
Pro Tip 1/3Slangy Expression
Considered informal and often used in casual conversations among friends, rather than in formal settings. 
At the family dinner, Aunt May said I was off my rocker for wanting to dye my hair green.
Pro Tip 2/3Hyperbole
It's an exaggeration used for effect, not to be taken literally as accusing someone of actual insanity. 
When Joe bought another vintage car, his wife exclaimed, 'You're off your rocker!'
Pro Tip 3/3Humorous Tone
Often used jokingly among friends to tease or mock someone's unusual idea or behavior in a light-hearted way. 
He's off his rocker, wearing shorts in the snow, but he loves it!