British and Australian slang, meaning good condition.

Examples (from google):

• "Wanted - Macpac Apollo Tent in good nick"
• "Absent Daffy says he's in good nick for debut" - The Sydney Morning Herald
• "like Directors or Ruddles - decent enough pints if you get them in good nick but shades of their former glories." - Realbeer.com

Not to be confused with Good St. Nick. 8^)