1. a set of rules or guidelines that are clearly defined or strictly followed
do what you think necessary, there are no hard and fast rules
A 'hard and fast' rule is a rule or guideline that is clearly defined or strictly followed. If you say there ‘are no hard and fast rules’ it means there is no guideline or strict rule to follow, you have some freedom to decide for yourself what the best course of action is. You can use this expression to make recommendations of what someone can do in a particular situation. ‘There are no hard and fast rules, but usually, the best approach is to...’ You can use this expression in a number of contexts, both social and professional contexts.