“Hard and fast” did not start out as a general expression about rules. It comes from older nautical language. In sailing, something described as hard and fast was firmly secured with ropes or fixed so tightly that it would not move.
From that literal sense, the phrase developed a more abstract meaning. Today, people usually use it to talk about rules, categories, or limits that are strict, fixed, or absolute.
For example, you might say, There is no hard and fast rule about how long an introduction should be. In that sentence, the phrase means there is no single rule that always applies.
It is often used in negative statements because many situations are flexible rather than absolute.
- Literal older sense: firmly fixed or tightly secured
- Modern common sense: strict, definite, leaving little room for exceptions
- Common contrast: a hard and fast rule is rigid, but a guideline is flexible
So the phrase moved from the physical world of ropes and ships to the everyday language of rules and decisions.