shrink?

[mrsmwallace]

If doctors are quacks then why are psychiatrists called shrinks???

[Ethel]

It's taken from 'headshrinker' because they mess with your head.

[Quizmonster]

To add just a little to Ethel's answer above...Primitive tribesmen in places such as Borneo used to decapitate their victims and - by treating them with hot sand etc - reduced the head until it was about the size of a fist, but still perfectly-proportioned. They were, therefore, literally, 'head-shrinkers'.
As a joke, someone considered that this was much the same procedure western psychiatrists used on their patients! They were, in other words, metaphorically 'shrinking heads'.
So, 'head-shrinkers' became shortened to 'shrinks'.