To be disturbed or disgusted at a gut level. Some people are squicked out by the thought of swallowing jizz, others are squicked out by the thought of men popping out babies, still others are squicked by the thought of licking an armpit. No one can judge another's squick factor, and what seems erotic to one person might totally squick out someone else.
Something a reader may find disturbing. For instance, blood, sex with an underage character (this is generally interpreted as under 18 years of age) or scat (using human excrement in a sexual way). Stories with squick potential are usually labeled as such (always on this site). It is considered the responsibility of the reader to avoid stories marked with elements that have a high squick factor since this is a personal judgment call.
Includes some gross concept or disgusting description that makes you go 'ewww!'
To do something which will intentionally gross out or bother someone.
really gross pairing(s) or content.
a term for something that grosses you out, especially in a fic. Squicks vary from person to person of course, but some examples could include foodplay, bloodlplay, golden showers, bestiality, etc.
To disgust someone with an activity past the limit of his/her tolerance; e.g. 'Watching that hard caning squicked me!'
Anything in fanfic which might make a reader queasy. What that might be varies between fandoms and even within mailing lists in a fandom. Character death is a major squick issue; sexual orientation, excessive violence or gore, or original female characters may do it for some.
A plot point, kink or device that turns the reader off or pulls them out of a fic.