Definition of "unprintable"

"There was poor old Wadley, of the Zoological Institute. Wadley sent a message: 'The President of the Zoological Institute presents his compliments to Professor Challenger, and would take it as a personal favor if he would do them the honor to come to their next meeting.' The answer was unprintable." "You don't say?" "Well, a bowdlerized version of it would run: 'Professor Challenger presents his compliments to the President of the Zoological Institute, and would take it as a personal favor if he would go to the devil.'"

1912, Arthur Conan Doyle, The Lost World […], London, New York, N.Y.: Hodder and Stoughton

Once a neighbor complained that a Gilbreth had called the neighbor's boy a son of an unprintable word.

1949, Frank B. Gilbreth, Jr., Ernestine Gilbreth Carey, Cheaper by the Dozen

LabVIEW will interpret one- or two-character codes following the backslash character, as shown in Table 2.1. You can also enter unprintables by simply typing them into the string.

2019, Richard Jennings, LabVIEW Graphical Programming, Fifth Edition, page 91