Doubloons were an old Spanish coin.
Mikey finds a doubloon with the map in his attic, which is inscribed with the year 1632 and has three small holes in its face. When Mikey, Mouth, Data, and Chunk approach the Lighthouse Lounge, Mikey is able to confirm the starting point of the map by lining the holes up with two rocks and the lighthouse in the distance.
Chunk drops the doubloon after being interrogated by the Fratellis. Jake and Francis confirm that it appears authentic, not merely a child's toy, suggesting that Chunk was telling the truth about One-Eyed Willy's treasure.
- Doubloon at Wikipedia