The smacking sound one makes with the mouth when displeased or irritated with something or someone. Known as "smacking of the lips" but the actual sound leaving the mouth is "smch"
"SMCH! He tried to tell me that my tomatoes were rotten when I know good and well they're fresh!"
Mom: "No, you can't go to the park to play ball today because we have to go buy you new school clothes."
Son: "smch. Do I have to go?"