The feeling of tiredness or sleepiness you get after eating a large amount of food.
Related to the fact after a snake eats a meal it much sleep to digest.
Dude 1: Awww man I ate so much I have black snake syndrome

Dude 2: what's black snake sydrome?

Dude 1: I've eaten so much all I want to do is sleep
by Whimsythemoondreamer March 31, 2013
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