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1.
To fall behind; someone who is mentally slow trails a lot... someone who says something dumb or irrelevant is trailing. Trailing can also be used anywhere where you would use the words crazy, tripping, or trippin'. Trail can be used anywhere where you would use the word trife.

Trailing can also be written or said as trailin'.
Adam: I like cows.
Tim: Dude, you must be TRAILIN' (trailing)!

Lisa: That new pair of shoes is totally trail!
by Klienmahn November 10, 2007
 
2.
Slang for smoking the prescription drug Oxycontin. The drug is smoked by splitting up the pill, usually in fourths, then placing a piece on foil. The pill is then slid slowing down the foil while following it underneath with a lighter. The smoke or gas released is then sucked up with some sort of straw. A black trail is left on the foil where the drug was being lit hence the name "trailing."
"Lets go trail some oxy brah."

"We're trailing the rest of that bottle tonight."
by SilentJ3737 July 09, 2009
 
3.
A term used to describe seeing a moving object not as an individual entity in motion but in serial, momentarily stationary positions. If the observer moves his finger across his field of vision, he not only sees his finger moving as a single object, but also sees the various individual movements needed to make up the complete movement. It is like a slow-motion multiple exposure effect.
Damn dude, That acid had had me trailing like crazy last night!
by Jay 081591 January 15, 2009