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1.
When the decibel level on a person's voice is raised to talk in an outdoor environment.
Sorry I can't hear you with all this traffic going by, use your outside voice.
by systemsengineer June 08, 2009
 
2.
When you say something that you were thinking without realizing it, usually something that the other person should not be hearing.
Before I could stop my outside voice from kicking in, I told Alison that I'd like to run my tongue threw her massive cleavage.
by grisgris1 February 24, 2011