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(pronounced like the first syllable in 'open') A massive joint, spliff, doobie, blunt, or other, preferably rolled as cone, containing some combination of weed/hash and opium.
"Yo dog, roll up a fat op cannon"

A: "Damn yo, I'm fuckin faded, whats in this?"
B: "Damn straight yo, you're puffin on an op cannon"

A: "wooorrrddd..."
by pumpkineater February 20, 2014
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