My Dominican friend said "Pa que no sepan". What does it mean?
The translators I use don't translate "Pa que sepan" or "Pa que no sepan". What is the meaning?

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Pa que sepan = Para que (ellos) sepan.
Pa que no sepan = para que (ellos) no sepan.
Pa en lugar de Para es un muy, my coloquial, mejor utilizar para.

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Depends... 
Pa que sepan= So they know or So you know (plural)
Pa que no sepan = So they won't know

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Gracias a todos!

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Yes the literal translation it's: so you/they know. But coming from a dominican guy he is using it more like slang, something you say after you do something cool or say something deep.

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Pa = (para)

Para que (ellos o ellas) no sepan (verbo saber).