Extremely secret. Of the utmost secrecy or top secrecy.(adjective)
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Use "ultrasecret" in a sentence
"Prosecutors said Gen. Gary Prado exchanged "ultrasecret" encrypted e-mail with Rozsa."
"Former Gen. Gary Prado allegedly exchanged "ultrasecret" encrypted e-mail with Eduardo Rozsa, a Bolivian-born Hungarian who was slain in an April 2009 raid by an elite police unit."
"The book also drops a hint as to the identity of another ultrasecret numerology sect that fascinates me, but I can't reveal their name here without ruining much of the surprise of the next book."