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1.
the maximum size your penis can be at any given moment. rock hard (happens rarely)
when you are extremely horny and aroused

"dude last i had the biggest boner, it was a megarection!"
by johnchulo August 05, 2009
 
2.
When a person becomes aroused, and their penis goes erect, and beyond hard. Swollen and massive, such an erection takes longer than average to go away, and can often cause uncomfortableness with the bearer. However, the Megarections should be worn proudly, in all situations, despite cultural or social norms
"So, last Friday I was struck by a huge boner in the middle of class...we're talking GIANT"

"Yeah, a Megarection. Get those all the time"

"What? I was so embarrassed!"

"Dude, rock that shit!! MEGARECTION!!!"
by GTPquickRB April 18, 2010