A person in America (possible including North and South America, but more specifically the United States) who partakes in the consumption of marijuana by smoke, vapor, or ingestion (IV too, but felt ridiculous to include.)
So, in summary: A person in America who partakes in any kind marijuana use.
"Is Obama Merican?"
"So, since Clinton probably ate brownies when he 'Didn't inhale', is he Merican?" (yes)