The act of breaking off a square of foil, breaking off a piece of heroin, setting it on the foil, and heating it just enough so that it will stick to the piece of foil. When using powder heroin, the act would be the same, except for you would pour the heroin onto the foil and then use your breath and fingers to heat the heroin into a small chunk. Often shortened to 'S a T'.
Alex: I'm ready to smoke!
Evan: Set a tray!
Alex: I don't know how... Can you S a T for me?
Evan: Only if you're splitting it with me.
Evan: Set a tray!
Alex: I don't know how... Can you S a T for me?
Evan: Only if you're splitting it with me.
by Cassandra Q October 21, 2011
Get the Set a traymug. 
by RodgerWilliams February 11, 2019
Get the Traymug.