Answer: hardandfastrule

AnswerCrossword Clue
HARDANDFASTRULEFirm principle
HARDANDFASTRULEIt's not meant to be broken
HARDANDFASTRULEStrict criterion
HARDANDFASTRULEThis is difficult to change
HARDANDFASTRULEThing not to be broken