What's a butfull?

When we had dinner at Kia & Ryan's last week, Daniel was experimenting with new vernacular. I love how his brain works and how he figures out new stuff. He was using the restroom and I took out a pack of toilet paper to refill.

He says "wow, that's a butfull of toilet paper!"
"What" I say
He repeats "That's a butfull of toilet paper."
I have him repeat this phrase once again, a little shocked at what he's saying.

Apparently, he heard someone saying "that's a butt load" of something and decided to incorporate this term into his vocabulary, though he was slightly off on the terminology. I stayed calm and instead of correcting him, I explained that this wasn't a nice way to talk. Once again holding in the laughter.