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1.
Noun: An involuntary erection whilst sitting fully clothed capable of casting a noticable shadow on the torso of the offender.
Having beers around the fire one night Quinton popped an absolutely absurd Shadowcaster.
by Alcatrazz69 October 30, 2012
 
2.
An erect (usually female) nipple that protrudes through clothing enough to cast a shadow in full light.
Dude, check out the shadow casters on that chick's fun bags!
by un_malpaso October 30, 2008
 
3.
A pest, a nuisance, someone who follows you around constantly, but poses no danger like a stalker.
That guy is such a shadow caster. Every time I turn around, there he is...all up in my face.
by Anonymous June 20, 2003