According to Gwendolyn Brooks the poem We Real Cool was written about the attitude and thoughts of a group of young men who skipped school to play pool at a local pool hall. This was an actual occurrence witnessed by the author and she was curious as to what went on in their heads, sparking her to write the poem.
In the line We sing sin I think she is referring to their character, meaning they are enthusiastically, without shame, performing deliberate acts against their own morals.
For example, I think she is implying by the line We thin gin, that they drink their parents alcohol and replace it with water in hopes they wont notice.
When reading this poem, before I knew that it was written in 1959, I picture preteen boys straight out of the movie Greese which debuted in the late 70s. I imagined them wearing leather jackets and their hair slicked back into a pompadour, with one cigarette lit in hand and another tucked behind their ear.
The setting pictured was a smoke filled, dimly lit dive where they stood arms crossed and heads held high. Their persona reeking of the Im better than you attitude as they nodded to all the passer-byers.
Unfortunately, most of us go through that rebellious stage during the early teenage years. We think skipping school is the cool thing to do, and we ignore our parents wishes and remain out past our curfews just to keep it real with our peers. Hopefully, we all outgrow this phase, but there are some who dont.
Those are the ones who continue to live carelessly and will most likely live a short life. I think this poem importantly reiterates, no good comes from bad and continuing in the lifestyle portrayed could lead to an early death.