Yours, Etc.: Origins and Uses of 8 Sign-Offs
1866, from intro- + stem abstracted from projection. In philosophical and psychoanalytical use, from German introjektion.
introjection in·tro·jec·tion (ĭn'trə-jěk'shən)n. The process of incorporating the characteristics of a person or object unconsciously into one's psyche, often as a defense mechanism.