thunderpants face

[frindle]

Hello. I'm not sure what 'thunderpants face' means in this context.
I read about the word, thunderpants, from Urban Dictionary:
a person who fart very loud.
and I also read that this word is from the English movie, Thunderpants.
But I don't quite understand the word in this context below.
A person who makes a display of farting loudly

What do you think?

----------------------------
"God knows what she's up to. What if she gets pregnant with that lad?"
Pregnant?! But I forgot the silent button.
Dad comes out into the corridor and sees me at the top of the stairs
and say, "You'd better come down."
So I stomp downstairs with my thunderpants face on and yell at Mum.

[5jj]

[I can only guess that it means that he had the expression on his face that normally preceded a display of flatulence.

[waflob]

I don't think it's a common idiom. From the context, it seems that 'she' is the narrator and that she simply has an expression of anger on her face.