Crazy or insane. I think you're off your nut for changing careers this late in life, but, hey, follow your dreams.He's off his nut if he thinks he can get that engine fixed by this weekend.
off your nut
out of your mind; crazy. informal
ˌoff your ˈnut
(British English, informal, spoken) crazy: Is he completely off his nut?
Nut is an informal word for your head.
off one’s nut
1. mod. crazy; out of one’s head. Don’t pay any attention to her. She’s off her nut.
2. mod. alcohol intoxicated. Those guys are so off their nuts!