When you do something you know you're not supposed to do (whether it's good or not so good) or when you do something that other people (like your parents) don't want you to do.
Well...sneaking out to Caid's party at 11 p.m. was a real ninja mission.
by avtor17 February 7, 2018
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Any secretive adventure legal or not, good or bad.
Damn my in-laws are visiting again! Gotta go on a ninja mission to get high!

Sarah just got a new girlfriend. She works at the mall. Ninja mission?

Marah just had her baby, let’s go on a ninja mission tonight and drop off some food!
by YubNub83 September 5, 2019
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