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1.
When you're taking a bath, jacking off and then bathing in your own semen.
"Goddamn it David, your masterbathing clogged the drain again!"

"Yeah, Kirk seems like a smart guy, but he's just too busy masterbathing to actually teach us."

Masterbathing is more private and intimate than dunk-tank-wanking.
by Tha 69 Boize January 17, 2013
 
2.
The act of bathing thoroughly and for a lenghty period of time.
If you are in the shower for more than 45 minutes, you are masterbathing.
by CaptainOJ May 23, 2007