The phrase “back of his hand” means to know something so well that you can recall it instantly and effortlessly, without having to think about it.
The origin of this idiom is not clear, but it is believed to have been derived from the practice of hand-to-hand combat where fighters would use the back of their hand to wipe away sweat from their forehead. This action would indicate that they were relaxed and confident in their abilities, and thus had a thorough understanding of the situation.
- She knew the city like the back of her hand.
- He had studied for the test so thoroughly that he knew the material like the back of his hand.
- After years of working in the industry, she knew the ins and outs of the business like the back of her hand.