A rusty skillet is when you cum inside of a woman that is on her period, leaving her insides coated in a pink mess, resembling the rust in an old, uncared for skillet.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

When you cum inside a woman who is having her period, leaving a rust colored mess inside of her, like the base of a neglected skillet