If something is sleetiest, it means it has the most sleet on it, like a road or a car.
The word sleetiest makes people think of cold and wet weather.
Examples:
The sleetiest road is very slippery.
Her car was the sleetiest of all.
The sleetiest day of the year is always so cold.
The sleetiest spot on the road is where the cars always crash.
In the sleetiest places, people have to be very careful not to fall.
The sleetiest hill is the most dangerous to drive on.