When vocalized or pronounced, it is similar to the sound of hacking up phlegm, but with a little bit more of a sibilance at the beginning of the sound-effect, and less of the usual hard 'h' sound.
This soud-effect is sometimes used in reply to seeing or hearing something that would, in the viewer's opinion, lead to or be worthy of any of the aformentioned sex-acts.
Ollie: Squick! Squick-squick!
Fortinbras: Aw, nerts. The sound's out on the telly.
Magda: No problem! I'll supply the dialogue! Squick-squick-squick...
Fortinbras: Hey, that's sick!
Magda: Come on! You've never thought Mary Tyler Moore needed a little more pizazz?
2. Noun. A situation or concept which engenders this reaction.
3. Verb, transitive. To cause someone to have this reaction.
4. Verb, intransitive. To experience this reaction.
The concept of the "squick" differs from the concept of "disgust" in that "squick" refers purely to the physical sensation of repulsion, and does not imply a moral component.
Stating that something is "disgusting" implies a judgement that it is bad or wrong. Stating that something "squicks you" is merely an observation of your reaction to it, but does not imply a judgement that such a thing is universally wrong.
The statement "kiddie porn squicks me" and "kiddie porn disgusts me" may both be true. In my case, the second sentence is true, and I assume that the first is also true, but, having never encountered it, I have no way of knowing for sure.
In general, distinguishing between "squick" and "disgust" is an important part of living in a tolerant society.
It is my contention that most anti-gay attitudes, for instance, are the result of people finding that gay sex squicks them -- and, because they don't know about the concept of the "squick", they assume that gay sex DISGUSTS them, which implies that there is something morally objectionable about it.
Somewhat similar to ick, but implies more disgust than simple distaste -- something that elicits the gag reflex would also be worthy of a "Squick!"
For example, if presented with a plate full of broccoli for dinner, one might say, "Ick." One would say "Squick!" if this broccoli had been at the back of the refridgerator for two months and was now moldy, slimy, and rotting.