D and T are the fourth and twentieth letters in the alphabet, thus, DT stands for 420. It is commonly used to cover up when one or a group are about to smoke and there is heat*(see "heat") around, such as parents, teachers, cops, and employers. If questioned, it can be easily covered up as slang as well. (downtown, etc.)
"Yo, i'm hurtin'. Time for DT?"
"Uh, let's go sm...DT."
December 05, 2006