Poop Sac

How can I be mad at a giggling kid? I sent my kids to bed twenty minutes ago, but the three younger ones (ages 4,7 and 11) are all snuggled up in Tom’s bed and are laughing and chuckling. I walked by and heard a strange expression being thrown around— “poop sac”. I don’t know exactly what a poop sac is, and I’m sure the kids don’t know either but they think it’s hilarious
(I guess it is a funny expression) .
I had to go into Tom’s bedroom and actually say to them, “no more laughing or having any fun, it’s bed time now, stop saying poop sac. I mean it, stop laughing, Ok guys, that’s enough, stop tittering, that means laughing, stop it and go to bed, really it’s bed time, I mean it.” I walked out and found my hubby in the hallway also snickering at the kids utterance of poop sac. Now I know where they get it…