"caring only for self; characteristic of one who cares only or chiefly for his own personal pleasure," 1630s, from self + -ish. It is common in Baxter and said by Bishop Hacket ("Scrinia Reserata," 1693) to have been coined by Presbyterians. 17c. synonyms included self-seeking (1620s), self-ended (1640s, from self-end, "personal or private object"), and self-ful (1650s).
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.