If something is seamiest, it is the most unpleasant or dirty.
The word seamiest has a bad feeling. It is usually about things that are not nice.
Examples:
The seamiest part of town is where the bad people live.
That is the seamiest shirt I have ever seen.
The seamiest alley is where the garbage is.
The seamiest food is the kind that fell on the floor.
The seamiest movie is the one with all the scary monsters.
The seamiest place in the house is under the bed.