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A letter that is typically anonymous and done as a joke or prank. The message is usually wacky or makes little sense Why are we getting so many annoying crank letters in the mail all of a sudden?
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
Also, crank call. An irrational, fanatical, or hostile letter or telephone call. For example, The office was flooded with mail, including a lot of crank letters, or Harriet was upset enough by the crank calls to notify the police. This expression employs crank in the sense of "irrational person." The first term dates from the mid-1900s, the variant from the 1960s.
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.