I guess he means we're going to be ghosts before he's through with us.
3 seconds sound clip from Scooby-Doo, Where Are You! - Go Away Ghost Ship (S1E14).
You can hear this line at 00:13:32.544 in the Blu-ray version of the episode.
Quote context
[...]
- There they go! Keep paddling, Scooby!
- Keep paddling and quit clowning around!
- Ha ha ha! Soon you shall join my crew! Ha ha ha ha ha!
- I guess he means we're going to be ghosts before he's through with us.
- I hate to spoil his fun, but we're gonna bust out of here!
- But how? The key for the lock is on the table... way over there.
- Like this... a pole made from this straw... and a little chewing gum... is going to get us the key!
- It won't quite reach. One more piece of straw ought to do it. Got it!
- Let's get out of these creepy caves!
- You said it! That seagoing spook is liable to show up any minute!
[...]