To be "on the natch" is to be in a natural state, without being intoxicated by drink or drugs. "Natch" being a shortened version of natural and "On the" being a prepositional prefix.

In context, to be on the natch would be going on a night out sober.
"Hey you out tonight?"

"Yeah, I mean I've got no money so I'll be on the natch tonight, but I'll hang out with you guys."

"No one has ever enjoyed your company sober, you are a depressive, drag on all our buzzes and we all wish you would die."
by LeastGhomie June 10, 2013
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To get yourself high by squatting low, breathing in and out deeply and quickly for about 30 seconds, and then standing up as fast as you can and blowing onto your thumb without letting any air out for about 15 seconds. Most of the time it knocks you out and you wake up feeling like you're in Inception. Sometimes it just gets you really, really high for a minute or two. Do not try this at home when your parents are there!
Dude, are you ok? You just passed out!
--Yeah man I was natching try it it's fun.
by PopnFreshKev August 31, 2011
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To chew noisily with your mouth open...
"Who's Natching in my ear?"

"Do you have to Natch when you chew?"

"Dude, stop Natching. It's really gross"
by MissCalissa December 10, 2015
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natch is a synonym for naturally.
"Bro i just bagged a sweet honey at the movies"

"Natch"
by -sexy man January 03, 2010
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shortened for "naturally"
Jon: Are H&M and American Apparel wonderful?
Kayla: Natch
by jl12181 September 11, 2008
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