The English word "Fiduciary" comes from the Latin fides (faith) and fiducia (trust). (Wikipedia).
A fiduciary has engaged in a relationship with his/her client that includes a trust and obligation (duty) to act only in that client’s best interests – without gain or benefit to the fiduciary.
Under the US Governmental structure: We, the citizens of the USA
By virtue of being elected – and hence having our trust placed in him/her, he/she is obligated to use an ethic of duty and care on our behalf in all his/her official actions. Period.