He was off the deep end now. He has been acting very strangely recently. He was thrown in at the deep end. He had to start doing something without any training or experience. He went off the deep end. He lost his temper. He went off the deep end. He started to behave in an unacceptable, probably antisocial manner. He went off the deep end. He is jumping off the edge of a swimming pool into the water at the deep end of the pool, where it will be over the swimmer's head.