The past participle of the verb 'superlay', which means to lay something on top of something else.
This word is not very common and might sound a little strange to some people.
Examples:
She has superlain the blanket on the bed.
I superlain the book on the table.
The cat has superlain on the warm blanket.
The dog has superlain on the couch again!
He has superlain a super heavy rock on the ground.
The workers have superlain the bricks on the road.