sight unseen - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Without having viewed the object in question, as in He bought the horse sight unseen. This seeming oxymoron—how can a sight, which means something seen, be not seen?—dates from the late 1800s.