Dear people of the English-speaking world, mostly Americans, I'm guessing:
The word "bad" is NOT a noun!
You can have a bad day.
You can have bad hair.
You can have a bad hair day.
You can have a bad thought.
You can have a bad child.
You can have a bad thought about your bad child.
You can have a bad relationship, a bad marriage, a bad temper or a bad wardrobe.
There are bad smells, bad choices, bad music and bad movies. Lots of bad movies.
What does the word bad have in common in all these sentences? In each one, it is modifying a noun. Therefore, the word "bad" is an adjective.
You cannot have a bad. Do you see how ridiculous this sounds?
The next time someone says to me, "Oh, my bad!", I think I'll respond with "Your bad what?"
Do you think they'll get it, or will I get a blank stare, and a "Huh?"
Yeah, I'm voting for the blank stare too.