rob someone blind

Cheat someone in an unusually deceitful or thorough fashion, as in The nurse was robbing the old couple blind. This idiom may allude to robbing a blind beggar, who cannot see that the cup collecting donations is being emptied. [Mid-1900s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.