Cause it's hotter than a lunatic's underarms in a straitjacket
In other words, we get it cracking, but I keep it cool,
Lunatics normally have erratic behavior and can sometimes harm someone else or themselves, so they are put in a Straitjacket to restrain them. “In otherwords, we get it crackin' but i keep it cool,”. “Crackin” can also be used to describe a unpleasant stench, but Kendrick keeps it cool.
This line could also be a reference to crackheads as they can go crazy and be put in straitjackets, as well as someone who’s gone crazy is sometimes referred to as a “crack.”
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