When you send someone a naked picture of themselves that they don't know you took. Especially useful as a pick-up technique.
"Dude, Anna just sent me a ninja sext!!"

"What do you mean?"

"She just sent me a picture of MY penis, but I've never been naked in front of her? WTF??"

"True ninja broseph. She's a ninja sext master."
by Ninjasextmaster69 May 23, 2011
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