Where unskilled skaters couldn't figure-8 twice
Figure 8 twice is 8*2 = 16. 16 bars is the standard length of 1 verse . So this line basically means most other rappers can’t rap, the rappers being called “unskilled skaters” as reference to the man-made lake mentioned above .
Another interpretation is that Unskilled skaters are crack dealers, who don’t have a craft or trade skill. After cyrstalizing the coke into crack rock, they were unable to get anything more than two “eight balls.” (an 8th of a gram).
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