means The quality of caring chiefly about your own wants and advantages while ignoring or undervaluing the needs of others.

from Built from "self" plus the suffix "-ish" plus "-ness," the word "selfish" was reportedly coined in the mid-1600s by English Presbyterian writers who needed a sharp name for the sin of self-regard — "self" itself reaches back through Old English to a Germanic root for one's own person. So the abstract noun "selfishness" is the youngest layer: the disposition named, then nounified.