The superlative form of 'sooty', which means covered in soot or very dirty from soot.
Being sooty is usually not good because it means something is dirty or covered in black stuff.
Examples:
The chimney was the sootiest part of the house.
After playing outside, my clothes were the sootiest.
The sootiest room in the house was the kitchen.
The cat came home looking like the sootiest cat in the world!
The sootiest chimney in the town was the tallest one.
The sootiest place in the whole world is the old factory.