So cold I done froze my butt off
And this ain't even the tip of the iceberg yet
A lot of things going on here:
First, notice his usage of two (plus one more below) very common sayings in English.
More importantly, this is a continuation of the ‘switch’ metaphor; Em becomes cold-hearted when he writes his lyrics, referencing his lack of sympathy and consideration for others.
However, there’s still a long way from his bottom up through his body to his head, which is why he annexes the idiomatic expression tip of the iceberg. This is a metaphor for the perpetual nature of his rapping: spewing out more and more great insults as the song progresses.
Furthermore, and this ain’t even the tip of the iceberg yet is Em’s warning that we haven’t yet seen seen the furthest extent of his abilities throughout his career.
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