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The unexpected,unprovoked erection that occurs while in public. Usually characterized as coming out of no where, particularly in the morning (at work or in class)

Scholars maintain that it a
reverberation of morning wood...
similar to an after-shock associated
with earthquakes

Morning wood is to ninja wood as
Earthquake is to after-shock.
This morning, in my math class, I had a mean case of ninja wood that came out of nowhere. (It snuck up on me like a stealthy ninja)
by Chris A. P. February 07, 2007