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1.
An individual who seeks food in any situation where no exchange of capital is needed, e.g., dumpster diving or crashing a wedding buffet
As an opportunivore, Lance preferred sponging off of friends rather than braving the cold, wet dumpsters.
by Dr. Octopus December 18, 2002
 
2.
Someone who selfishly exploits or takes full advantage of any situation involving free food or items.
The opportunistic omnivores in the office will eat anything we leave unattended in the kitchen, and now those opportunivores have eaten all the Christmas cookies.

That opportunistic jerk took all the free condoms from the clinic, they're such an opportunivore.
by Knickerbøcker November 30, 2009
 
3.
Someone following the "high-everything diet", i.e. high-fat, high-carb, and high-protein, without any restriction on calories or macro-nutrient ratios. Could be a competitive eater, big body-builder, or just a normal person who will eat anything.
An opportunivore will eat whatever is available.
by rensielk March 27, 2009