look up any word, like darude - sandstorm:
 
1.
-An expression used to indicate to the receiver that theirs gears (like on a car) are too high, thus they need to relax, slow, chill out, gear down.
Stephan: OMG MAN roll that joint, get some food, man lets go over there, hey hand me your lighter, wheres that L? Liam likes boys..

Dev:Alright man you just gotta gear down...
by Linkincide August 07, 2008
 
2.
A term used for a person who suffers from the mental disorder known as egoistic-ism. Also known as a person who obseses about themselves and needs to be put in their place.
Bob: "Woah, dude she won't shut up, she keeps going on and on about that dude"
Bill: "Actually? she needs to gear down.
by Scotty T. April 14, 2011
 
3.
To relax, or to calm down; usually used as the "last word" in a conversation; usually used to take the tension out of a situation, causing each person to smile and realize they are being an idiot.
Examples of usage:
Example #1:

Jonathan: "Sarah, what did you do with my bib shorts? I told you I needed them for today's ride!"
Sarah: "Gear down, they're in the dryer."

Example #2:
Guy #1: "What's up with your outfit? Did you get dressed in the dark?"
Guy #2: "Gear down."
by Lucy13 August 01, 2009