We them pro parade-rainers
Raining on someone’s parade means being critical of something. P.O.S. is saying Doomtree is willing to be that critical voice that may wreck the party, but will speak truth and opposition to popular ideas. In a parade, large crowds gather around to watch floats and people march down main streets. There are often patriotic tinges to parades — celebrating the history of a country, the accomplishments of the country, and the country’s overall excellence. There are several acts of pretending involved:
1. participants go to great lengths to appear happy and excited
2. viewers are expected to be pleased and cheer
3. everyone is pretending that the country is what they are portraying it as, which involves self-deception as a nation. there is a disconnect between the reality and the nation’s mythical image.
Parades also have a militaristic feel as they are reminiscent of soldiers marching in lines. P.O.S. is saying that Doomtree, as a group, is going to cut through what he considers to be nationalistic, over-patriotic bullshit and be real. He’s also encouraging his listeners to rain on parades a.k.a. speak critically and honestly to misguided half-truths — he says Doomtree members are pro parade-rainers.
This line could also refer back to the line “They make it rain, rain, rain go away.” P.O.S. is contrasting what he and Doomtree are in favour of (making it rain on parades) with what other rap artists encourage (making it/money rain on women).
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