means The quality of putting others' needs and well-being ahead of your own, without expectation of reward or recognition.

from A transparent English build: "self" (Old English self, the same word that's anchored our sense of personhood for over a thousand years) plus "-less" (Old English -lēas, meaning "without," a cousin of the verb "lose") plus "-ness," the noun-making suffix that turns a quality into a thing. So literally: the state of being without-self. "Selfless" appears in English by the early 19th century, with the abstract noun following close behind.