introvert (v.)

"turn within, direct inward," 1650s, from Latin intro "inward, within" (see intro-) + vertere "to turn" (from PIE root *wer- (2) "to turn, bend").

1878, in zoology, "part or organ which is turned in upon itself," from introvert (v.). The psychological sense "introverted person" (opposed to extrovert) is 1917, from German, introduced there by C.G. Jung (1875-1961).