A combination of the words Mason and Upshaw referring to a very large and erected penis that is present in a state of drowsiness combined with a sense of stupidity or lack of intelligence. Mason Upshaws usually occur when one has just woken up and watches and excessive amount of pornography or goes out on the street and sees many acceptable women. Also called extreme Morning Wood.
Dude: Aw crap I woke up late for class again...and I'm too sleepy to go...guess I'll watch some porn.

After seeing first image of nude woman

Dude: Holy shit! I've got a massive Mason Upshaw!
by Master Alchemist July 27, 2011

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