hard-and-fast

Adjective (来源：英语杂志 http://www.EnglishCN.com)

strictly enforced (as of rules)
to stick to hard-and-fast rules

不容变更的;严格的
His parents have no hard-and-fast rules about how late he can stay out.
他的父母对他在外可以逗留多晚并没有严格的规定。

He stuck hard and fast to his principles.
他严守自己的信条。