Better watch your back and duck
Cause these fiends, they got it cracked up
If you have a lot of money you should watch your back because it is very likely that you will be robbed by people addicted to hard drugs (fiends) who would use it to purchase said drugs.
I think this is also a pun. He means crack fiends are crazy (“cracked up”) enough to try and mug you for drug money, but also they’re crack fiends so they’ve got it “cracked up”.
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