Benevolent, honorable behavior considered to be the responsibility of persons of high birth or rank.
Wiktionary
(1 definition)
–noun
the honourable obligation that is the responsibility of those of high rank; in American English this often includes the expectation of benevolent actions such as helping those less fortunate.
WordNet
(1 definition)
–noun
The obligation of those of high rank to be honorable and generous (often used ironically).
"Sometimes, there is no choice, noblesse oblige and all that."
—The Last Gamble
The main salon was the venue for the Cowper Foundation's scholarship presentations, an annual exercise in noblesse oblige whereby the tip of Oakleigh's social pyramid dispensed tokens of grace and favor to a few young but deserving denizens of its lower tiers.
—Alfred Hitchcocks Mystery Magazine
Each year, as the Prince reminded her, the Queen wondered if it was worth provoking a constitutional crisis by simply refusing to recite the nauseating drivel, but the Queen knew she couldn't, noblesse oblige and all that.