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1.
1)Third gear is another way of saying mackin', or pimpin'. like- 3rd gear in a mac truck.

2) Third gear is that stage when a male and female are inbetween being just friends and "officially" dating

3) It may be used to describe overdone affection between a dating couple

4) It may be used as a synonym for girlfriend or boyfriend.
Noun--
1)Hey, are you in third gear with Lindsey?
2) Greg, where is your third gear?

Verb--
Wow! Matt and Jamie are really third gearing it.
by Melizabeth Butler April 18, 2008