How Kids Learn Bad Language

We were flipping through channels, when we came up on an old comedy- Life! Starring Eddie Murphy and Martin Lawrence. It was actually an all star cast! Well being that it was during the day and on regular television and not cable we really didn't think much about watching a little bit of it in front of our son. We just didn't think. Have I said that already?
Well when we tuned in it was right after the prisoners were loud from laughing about Ray's (Eddie Murphy) descriptive story of Ray's Boom-Boom room back in Harlem. Then the guard comes in with his lantern, and tells them to get quiet- well he actually tells them to, "Shut their mouths!" Then he turns to walk away and one prisoner (Anthony Andersen) passes gas. Well had we remembered we would have turned it then. Well if you know the movie you know what happens next.
The guard turns to respond to this passing of gas and says, "Shut your mouth and your fat a##!" (This is a family blog)
As soon as he said it, our laughs halt, Daddy flips the channel and our son laughs and repeats what he (guard) said- all in that quick moment.
Daddy and I exchanged a quick glare. We didn't respond immediately.
"Hey, I was watching that!" our just recently turned, six-year-old son said.
Daddy and I looked at each other to see who was going to respond first.
"You can't say that. That is a bad word," Daddy told him.
He was confused.
"What's a bad word Daddy?" he asked.
"What that man just said. Then you said too," I told him.
He sat and processed it.
"You can't say everything you hear someone else say. This is why we don't like for you to watch so much television and especially adult shows."
We should have gone over it a littel bit more.
A day later (Sunday of all days), we are eating at the table and he repeats it.
He and his sister are doing what they do best- teasing each other.
Then he tells her, in between bites of chicken, "Shut your a$$ up!" and laughs as if there was nothing bad about it. But I guess it should have been that way because he and his sister do not know what profanity is. Up until this point, he thought bad words were: shut it, zip it, shut up and stupid.
So it was another teachable moment.
"You cannot say bad words," we told him.
"What was a bad word?" he asked. His sister was waiting for the answer to that too.
"What you just said when you said something to your sister," I told him.
It was quiet for a moment. No one was laughing. The kids wheels were turning. Daddy and I were trying to figure out what to do next.
"I can't say bad words even when y'all aren't there?"
"No!" Daddy firmly told him. "God will tell us."
"He tell y'all everything?" he asked.
"Yep! How do you think we know about everything you do, even when we ain't there?" Daddy asked him.
"Oh. That's how you knew about me talking back to grandma?" he asked.
Before we could respond, his sister jumped in.
"Actually I told on you about that."
Daddy and I didn't know anything about that.
"And actually (looking around) he talked back to me too," his sister added.
We had to explain to her that she was not an adult so that would be different.
"Well actually I AM older than he is," she suggested.
At this point I felt that we were gettin away from the main point and needed to draw us back in.
"Well actually you can stop talking!" I firmly said.