What is "gag rein"

Gag \Gag\, n.

1. Something thrust into the mouth or throat to hinder speaking.

2. A mouthful that makes one retch; a choking bit; as, a gag of mutton fat.
   --Lamb.

3. A speech or phrase interpolated offhand by an actor on the stage in his part as written, usually consisting of some seasonable or local allusion. [Slang]

4. a remark or act causing laughter.

5. . A prank.

   Gag rein (Harness), a rein for drawing the bit upward in the horse's mouth.

   Gag runner (Harness), a loop on the throat latch guiding the gag rein.

n. A rein for drawing the bit upward in the horse's mouth.