A three letter word

The car was loaded with backpacks, lunch bags, jackets and buckled up children. I remembered something I had forgotten and dashed back in the house. When I returned a hush fell over the normally chatty, noisy car.
"Mommy,' K said very seriously, "P said a BAAAD word."
"He did?" I asked, giving P 'the eye.'
"Yep."
My curiosity was getting the better of me, so instead of letting it go I asked K, "What did he say?"
"I can't tell you 'cuz it was an ugly word."
I could only think of two words they have recently been cautioned not to use: stupid and darn.
"Did he say stupid?"
Her head shook side to side. P looked down at the ground, then glanced up to guage my reaction.
"P, did you say darn?"
"No."
I realize I have violated all kinds of parenting rules, but I was so curious about what new word they may have picked up (and where it came from) that I simply could not let it go.
"K, what word did he say? You will not get in trouble for saying it, since I asked."
"He said GUM!" she shouted with a glimmer in her eye. She searched my face to see what kind of reaction was coming. What she saw was relief mixed with amusement and confusion.
"Gum?"
She shook her head up and down, with eyes as big as saucers. She really wanted to see what the punishment for this crime was going to be.
P looked up again with wide eyes.
"Gum is not a bad word, sweetie." (Although I do not let the children chew it, so I guess she was equating off-limits to naughty.)
"Yes, it is!" K protested. "Remember when we watched that movie with the rooster man and the little girl? She was singin' that song about her gum dog and you told us never to say that word?!"
Ahhh. Now I get it. Through giggles I explained that the word Annie used was 'dumb' not gum. And, yes, she was right, there is no good reason for us to use that word.

All afternoon I sang "Gum dog, why are you following me?"

Next we deal with their tattling and my insatiable curiousity that makes mountains out of molehills...