According to TV Tropes, "Squick" is possibly a contraction of "squeamish" and "Ick!" A negative emotional response, more specifically a disturbed or disgusted one.
"Squicky" and "squicked" are also common forms, with that very wiki having apparently invented the form "squicktastic".
It is a theme of Random Klips that feature something disgusting in nature.
Klips with this theme[]
- Carmelita Ate One Too Many
- Conker and Banjo Grossed Out
- Nothing Bad Ever Happens to a Clown
- Marco's Fear of the Finger
- Phineas' Nightmare