420 Blaze It is a usual term used to describe the action of smoking marijuana. Frank describes this sentence as a very important phrase in litterature, although Frank himself is an asshole who doesn't understand litterature himself, since he dosn't even read litterature. He even says "maridjuanee" or "maridjuanal" instead of "marijuana" due to his filthiness. This phrase, like "I Eat Ass" and "Kancho", was also covered in a Japanese 101 episode. However, you can add the notorious "faggot" at the end, giving "420 BLAZE IT FAGGOT" or "420 BLAZE IT FGT" as a result.
Since marijuana is obviously illegal, Frank has decided to use some Malaysia Airlines, which is unknown to the authorities but it is deadly. After smoking a joint, Frank ended up having Pink Guy killed by a banana.
See also: Malaysia Airlines