crank letter

noun

1. a hostile or fanatical letter, often sent anonymously.

Idioms and Phrases

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.