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1.
Shortened from the word treacherous, tretch describes something that is dangerous and if desired, cool.

Much like the slang word "sick" it can be used in both positive and negative scenarios.

In the positive sense it can mean fun, daring, chill, etc.
In the negative sense it can mean sketchy, wrong, or terrible
For example:

Positive: "Hey Demarcus, that party was majorly tretch, mann!"

Negative: "Yo man, we have to get out of here. This shit is hella tretch."
by Treacherous Whore July 22, 2009
 
2.
Tretch is a furthur acknowledgement that something is Sketch(y) or Unreliable, It is also a combination of the word true, and sketch. And can be applied to any lame or Sketchy situation
" That police officer is going to see us smoking this sticky-icky bong rip!
This is Tretch Dude!!!!"

" That party was tretch man, there was alot of illegal activity and I felt threatened by it..."
by Sebastian Fox October 07, 2003