Plain old stupid
Something or someone who fits into the category of normal stupidity. Plain and old merrily describe the word stupid. The something or someone is not necessarily plain or old.
He couldn't use his printer because he didn't turn it on. He wasn't drunk; he was just plain old stupid.
The boy didn't pass his test not because he didn't study, but because he was just plain old stupid.
The boy didn't pass his test not because he didn't study, but because he was just plain old stupid.