What is the origin of the idiom "against the grain" and when to use it?

The idiom "against the grain" originated from the practice of milling or grinding grains, particularly wheat. When grinding wheat into flour, it is more efficient and easier to go with the natural alignment of the grain, as it requires less energy and produces a smoother result. However, if one were to grind the wheat "against the grain," going in the opposite direction, it would be harder and more challenging. It can be applied to situations where people challenge societal standards, question traditional practices, or choose their own paths.