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1.
the act of pleasuring oneself by using only the mind to concentrate on something stimulating.
No one would every know that during the meeting Brandon was mentally masturbating because he wasn't using his hands to touch his penis.
by Lateesha223 July 15, 2014
 
2.
Adjective: A state a mind where you seem aloof but brain function is occurring.
Just so you don't think I was mentally masturbating all day here is that excel spreadsheet you requested.
by patmyballs July 10, 2009