1630s, from self- + -ish. Said in Hacket's life of Archbishop Williams (1693) to have been coined by Presbyterians. In the 17c., synonyms included self-seeking (1620s), self-ended and self-ful.
Let us understand what our own selfish genes are up to, because we may then at least have the chance to upset their designs. [Richard Dawkins, "The Selfish Gene," 1976]
Related: Selfishly; selfishness.
Similar formations in German selbstisch, Swedish sjelfvisk, Danish selvisk.