To squick is to offend, surprise, exasperate, or shock any person (usually) not related to the furry fandom, with extreme displays of unacceptable furry behavior, which would sometimes include overt sexual acts.
These incidents normally occur during events where furs and mundanes come in contact with each other in large numbers. One example is furry conventions, where they must mingle in the same hotel facility hosting the event. Even though some furries take pride in "squicking the mundanes," this type of behavior is not condoned by the majority of the fandom, or convention staff.
The proposed unit of Squickliness is the milliWinger, named for furry artist Doug Winger. An alternative unit is the Shudder, equal to the number of milliseconds the victim shudders after encountering the squick-inducing image or concept.
 See also
- Squick on Wiktionary
(Please note that learning the actual origin of this word may squick some readers.)