The superlative form of the adjective 'silty', meaning having the most silt.
This word is used to compare things and say which one has the most silt.
Examples:
The riverbed is the siltiest place in the whole river.
This soil is siltiest after the rain.
The siltiest part of the lake is near the shore.
The siltiest mud makes the best pottery.
After the flood, the siltiest areas were hard to walk through.
The siltiest sand is at the bottom of the ocean.