The Brief: Down packed is a mispronunciation of "down pat." It means to have something rehearsed or practiced to near perfection.

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“Down packed” means an individual has rehearsed or practiced something near perfection, according to Urban Dictionary. It is a mispronunciation of “down pat.”
Took me a long time to learn this. I have it down packed now ππ pic.twitter.com/K9zC8V0AeI
— Lif3WithLoLoπ³οΈβππππ (@Lif3WithLoLo) March 25, 2021