Little girl panties that have been worn too close to a dusty, cobwebbed crotch for too long. If someone has Krusty Undies they should think about getting some big girl panties and changing them more often.
This term is also often used to describe an adoptophile, specifically a barren woman who chooses to adopt from countries rich in child trafficking or someone who acts in a demeaning way toward all adoptees except her own child.
"WOW! She sure put on her Krusty Undies when she told that adoptee to be grateful they weren't aborted or tossed in a dumpster."
When you wear your last chance undies two days in a row because you forgot to do laundry...again. They can be worn either in a normal manner, or inside out to produce a psychological effect as to thinking you actually have clean undies on; Or because you have skid marks and or piss stains.
Nigga to mom "THIS IS AN EMERGENCY! I GOT A DATE AND I HAVE ZOMBIE UNDIES ON B...ITCH!!"
Mom to Nigga "You disgusting fucking pig of a son!! I TAUGHT YOU BETTER! NIGGA!"