Originally: the noise made while skullfucking.

It has since been expanded to mean anything disturbing or distressing or disgusting that one would not willingly engage in. Often sexually related. As in, "Beastiality? That just squicks me. I'd never do that."

The brand name of a thick liquid product your pour into your hot water heater to make your pipes stop banging around in the middle of the night. Sort of like Draino for boilers.

Another use is the disturbance or perturbation of sheltered people due to one's speech or actions. An example would be certain people's reactions to my eyelet, although this is a very innocuous example. This can get far worse though; often explaining squick's other definitions does the job quite nicely.

This usage was apparently coined by SM enthusiasts; it has since spread to body modification enthusiasts as well.

The term has been adopted by the Furry fandom as well, the irony of which is left up to the reader.