Rhymes be the proof while I'm drinkin 90 proof
Alcohol proof is a measure of how much alcohol (ethanol) is contained in an alcoholic beverage. The term “proof” was originally used in the United Kingdom and was defined as 7/4 times the alcohol by volume (abv). The UK now uses the abv standard instead of alcohol proof. In the United States, alcoholic proof is defined as twice the percentage of abv.
Meth is saying that when he drinks his rhymes are still mathematically sound. i.e, proofs.
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