It's a group of islands. The Archipelago of Last Years is the most fantastic archipelago of all.
9 seconds sound clip from the Rudolph's Shiny New Year movie soundboard.
You can hear this line at 00:13:43 in the Blu-ray version of the movie.
Quote context
[...]
- Gee, I wish he would've come to me. I know just how he felt.
- I bet they called him names, and wouldn't let him join in any new year games.
- Do you have any idea where he went?
- His nurse, Nanny Nine O'clock said that he always wanted to see the Archipelago of Last Years.
- What does that word mean, sir?
- It's a group of islands. The Archipelago of Last Years is the most fantastic archipelago of all.
[...]