Not To Say Meaning

Example: Used other than as an idiom: see not,‎ to,‎ say.
Even; perhaps; almost.
c. 1962, Jacqueline Shohet Kahanoff (author), Deborah A. Starr and Sasson Somekh (editors), “Wake of the Waves”, in Mongrels Or Marvels, Stanford University Press (2011), ISBN 978-0-8047-6953-2, page 145:
  At first Marc was somewhat shocked, then he burst out laughing and finally came to the conclusion that actually it was all rather sad, not to say stupid.