Blake-ism

So we were getting the kids ready for bed last night. Blake was keyed up and was trying to put off bedtime. He walks into his and Cathryn's room and they start bickering (imagine that) about some odd thing or another. Next thing I know he's walking down the hallway and says:
"I don't want to sleep with my noxious sister."
I said "What?"
Blake: "I don't want to sleep with my noxious sister."
I said "Go tell Daddy what you said." (while trying not to laugh too hard)
Blake repeats: "Daddy, I don't want to sleep with my noxious sister." and walks to their room.
I ask Ken: "Do you remember what noxious means?"
Ken: "No."
I reply: "Harmful. bad, that sort of thing."
We both almost collapse from laughter.
Well, I looked it up and the dictionary defines it as " physically harmful or destructive to living beings <noxious waste> b: constituting a harmful influence on mind or behavior."
Does Blake know his sister or what? So what if he was trying to say obnoxious. It's the same thing as far as he and Cathryn as concerned.