Undercrackers?

I have never heard of this term before. I think it’s English.

Such drama a pair of one woman’s panties can cause! 

“like a red rag to a bull”  had Me laughing My ass off.

“she had all of her undercrackers stolen before”.

Undercrackers?  For some reason I find this term incredibly funny. Wouldn’t they technically be OVERcrackers……   LOL