A more than respectable size penis which, when in its erect state, is so engorged with blood causing the veins on the shaft to protrude and thrash about during intercourse. Hence, a vein thrasher.
Oh my god -look at his bulge! I bet you he has a vein thrasher under those khakis.
by J Bianchee-WideMountains April 30, 2010