Simply Abominable is related to the seasonal world event, the Feast of Winter Veil. Completing this achievement is a criterium for the meta achievement .
You can pick up the first quest from Kaymard Copperpinch in Orgrimmar or Wulmort Jinglepocket in Ironforge. Then go to the Alterac Mountains to find the Strange Snowman and defeat The Abominable Greench. Return to Orgrimmar/Ironforge to do the last quest and get the reward.
/ Patch 3.0.2 (14-Oct-2008): Added.