"Moral fibre" refers to someone's strength and integrity in sticking to their ethical beliefs and principles, even when faced with challenges or temptations to act otherwise. Full definition
Is the former more lacking in moral fibre than the latter? (newlawjournal.co.uk)
This lack of moral fibre makes for an interesting narrative. (hoopla.nu)
The statement that there is «honour among thieves» suggests that even within groups that might be seen as lacking in moral fibre, a certain ethical standard is upheld. (remonline.com)