The pun in the line "Will's skills are sick like ERs, you heard of this?" plays on the word "sick." In one sense, "sick" refers to being ill, which connects to Emergency Rooms (ERs) where people go for medical emergencies. In another sense, "sick" is slang for something impressive or excellent. So, the speaker is highlighting that Will's skills are both "sick" in the sense of being outstanding and also humorously implying that they are "sick" in a medical sense, relating to the ER.
When you make a pun, you play with words that have different meanings. "Going deep in double meanings like they were a coal mine"" is a play on the word "deep." What's the the pun in the following line? "Will's skills are sick like ERs, you heard of this?"
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