"What's a cripple?"

SO, yesterday, as we were leaving Lucas' school, one of the older classes and teacher were exiting the gym. As we walked past, the teacher made a "joke," because her students were apperently moving her along too quickly for her taste. As we walk by, she says, "Hold on, I know I cant move as fast as you, but it's not like I'm a cripple! Ha-ha-ha!" Lucas turns to me, holding my hand and says, "Mom, what's a cripple?" All big-eyed and innocent. I tell him, "Well, it's kind of a not very nice way of saying Handicapped." We recently had a discussion about what handicapped was, when he noticed the hanging placard in some cars, Like Grandma Kathleens, or Grannies.
When I told him that that was what he meant, He said, "You mean, like my Dad?" and I went "Yeah, I guess she wasn't thinking about what she said. Everybody forgets things sometimes."
I thought all was okay, until, about half way home, over the Radio Disney bumpin' through the car, (Part of our after school routine,) I heard him sobbing in the back seat. I ask what's wrong, and he says, though I'm paraphrasing, that his feelings are hurt because anyone would cal his Daddy any name, and that there is a bad name for people who cant do things, and why would that teacher say that, and it's not fair, and he loves his Daddy.
It's just such BS. Some people just don't think.