Fiduciary (adjective)

1. Involving trust, especially with regard to the relationship between a trustee and a beneficiary.
2. Relating to the responsibility to act in the best interests of another person or entity.

Origin:

From latin 'fiduciarius' ('involving trust'), from 'fiducia' ('trust').

Examples:

1. The attorney had a fiduciary duty to his client.
2. As a trustee, she had a fiduciary responsibility to manage the assets in the trust prudently.