Diagnosis
This is either a bent output shaft or a damaged spindle thread. Forcing a stuck nut or dropping the tool on the wheel can cause this.
How to Fix It
Confirm by removing the disc and nut. Spin the shaft by hand while watching the very end of the threads. If you see a wobble, the shaft is bent. Replacing the output shaft requires a full gearbox teardown and press work to remove the old shaft and bearing.
If That Didn't Work
The wobble might be from a worn spindle bearing, allowing lateral play. Check for up/down play in the shaft. If there's movement, the output shaft bearing is shot.
Heads Up
A wobbly wheel is dangerous. It increases the chance of disc shatter and causes uneven wear, overloading the tool. Stop using it immediately.