Mental health professionals are called "shrinks" due to the early use of the slang word "headshrinker," acting as a shortened form of it and displacing the word in common use over time. The word is used as slang that refers to their work with personality disorders and the mind.
The term "headshrinker" comes from the practices of the Jivaro natives, in which the heads of hunted enemies are removed and then shrunk through an extensive process. The Jivaro are made up of four different tribes, all of which practice this tradition in order to celebrate victory over their enemies and discourage further conflicts against them. This tradition remained effective even against those exploring Spaniards who coveted the lands the Jivaro tribes called home.
In the modern era, "headshrinker" is used to refer to all mental health professionals, whether they are psychologists or psychiatrists or belong to other professions.