The Only Use

Morse code
in
Turtles All the Way Down
(Auto-generated)

Her legs were crossed, and her left foot was tapping the ground like it was trying to send a Morse code SOS.†
p. 86.2

Definitions:

(1) (Morse code)
a code in which letters and numbers are represented by strings of short and long signals such as dots and dashes

Originally used for telegraph messages and still in specialized uses today where a non-computer assisted code is desired. Perhaps the most innovative use of Morse code is when an American prisoner in the Vietnam War was forced to give a delayed-broadcast television interview. Unknown to his captors, while talking, he blinked his eyes in Morse code to spell out the word "T-O-R-T-U-R-E" and other messages.
(2) (meaning too rare to warrant focus)