Reprobate
Should
???
Reprobate
Offensive
Not calling names is a good option.
Also I'm not sure reprobate and stupid are equal.
Reprobate
1 : to condemn strongly as unworthy, unacceptable, or evil
2 : to foreordain to damnation
3 : to refuse to accept
If you're going to call names there is no inoffensive side way to do so. Therefore no reason to try and pretend you're doing anything less. If you're going to insult someone be clear about it.