Look Out For Number One Meaning

(idiomatic) To act in one's own interests; to act in a manner advantageous primarily to oneself.

Example: 1870, Horatio Alger, Rufus and Rose, ch. 3:
  "I'm a poor widder, Mr. Martin, and I must look out for number one. I can't afford to keep boarders that don't pay their bills."
1906, Mark Twain, "What is Man?":
  Old Man: A man performs but ONE duty—the duty of contenting his spirit, the duty of making himself agreeable to himself. . . . [H]e always looks out for Number One.