This is a part from The Old Man and the Sea by Ernest Hemingway
There was no cast net and the boy remembered when they had sold it. But they went through this fiction every day. There was no pot of yellow rice and fish and the boy knew this too.
“Eighty-five is a lucky number,” the old man said. “How would you like to see me bring one in that dressed out over a thousand pounds?”
“I’ll get the cast net and go for sardines. Will you sit in the sun in the doorway?”
What does that dressed out exactly mean there?