We real cool. (line 1)
If that isn't pride we don't know what is. But what kind of pride? Arrogance? Cockiness? Self-assurance? Maybe their pride even makes the reader more inclined to empathize with them.
We Strike straight. (lines 3-4)
Their ability to hit a round ball into another round ball and get that second round ball into a pocket appears to be one of their best skills. Can you think of other meanings for "strike straight" that don't involve pool? What do these words suggest to you?
We Sing sin. (lines 4-5)
Walt Whitman was the ultimate poet of self-confidence, and he even had the gumption to say he celebrated himself. The pool players use one of Whitman's favorite words – to "sing" – but they are singing "sin" instead of themselves. Could their praise of sin be the result of a lack of self-confidence, rather than an excess of it?