ressenti(e) is the past participle of the French verb, ressentir, and ressentiment is the noun form. While ressentir can simply mean to feel inside, ressentiment refers to feeling aggrieved. Nietzche makes use of ressentiment constantly, in his own singular fashion, to describe the phenomenon whereby an active force is deprived of its normal conditions of existence, where it directs itself inward and turns against itself. "Pushed back and repressed, incarcerated within and finally able to discharge and vent only on itself." This is the perfect definition of what is meant for something to be ressenti according to Nietzche's concept of ressentiment.
For Wendy Brown, following Nietzche, the wounds that underlie the politics of identity lead to ressentiment, a powerless over the past— a past of injury, a past as a hurt will, as a "reason" for the "unendurable pain" of social powerlessness in the present. (see identity politics) —Wendy Brown, States of Injury
In his Nietzche and Philosophy, Deleuze defines ressentiment as the becoming reactive of force in general. "separated from what it is capable of, the active force does not how ever cease to exist. Turning against itself, it produces suffering." Hence, Deleuze concludes, with ressentiment a new meaning and depth is created for suffering, an intimate, internal meaning. (Anti-Oedipus, translator's note p. 214)