If something is skankiest, it is the most dirty, smelly, or gross.
The word skankiest is not nice. It makes people think of bad smells and things that are not clean.
Examples:
That is the skankiest sock I've ever seen.
The skankiest alley is behind the old store.
The skankiest dog in the park is always rolling in the mud.
I don't want to touch the skankiest trash.
The skankiest bathroom in the world is in that old house.
The skankiest food is always left in the fridge too long.