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1.
Small amounts of bile that comes into your mouth or back of the throat. The chances for this to occur increase after (1) eating or drinking acidic substances such as ornage juice (2) viewing or smelling something that would invoke the bodies gag reflex. Throw up junior is usually identified by a pungent, acidic taste in the back of the throat or mouth. Throw up junior does not involve the act of vomiting solids from the stomach.

This term was made popular by the movie super troopers. during the cast/writer commentary throw up junior is used to describe a scene in which Officer Rabbit experiences bile in the back of the throat while viewing a fellow officer being shot at in the testicles.
Justin: Ahhh I just had a throw up junior after drinking that orange juice.

by kcraigggg February 29, 2008
 
2.
When you vomit a bit in your mouth, usually followed either by:

1. Spitting out the stomach acid and chunks
or
2. Swallowing it like most lazy people do
Nick: "Oh sweet Jesus!"
Larry: "What?"
Nick: "Throwup Junior is having a family reunion in my mouth."
by RadioFiasco January 11, 2006