To confuse someone about which direction they are facing or where they are, or to make someone feel lost and unsure.
From dis- (opposite) + orient (from Latin oriens 'rising,' referring to the East). Originally, 'orient' meant to position something toward the East, but it became 'to position oneself' generally. So 'disorient' means to remove someone's sense of position.
Sailors in the medieval period would 'orient' themselves by facing East (where the sun rises), so disorient literally meant 'turn away from the East'—a directional term that became about confusion!
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