We wear the mask that grins and lies,
It hides our cheeks and shades our eyes,—
This debt we pay to human guile;
With torn and bleeding hearts we smile,
And mouth with myriad subtleties.
Why should the world be over-wise
In counting all our tears and sighs?
Nay, let them only see us, while
Deception has become so ingrained, it is part of the ways humans interact with the world of others. 
 
 Mouth  here is a verb meaning speak or talk, almost silently or muttered under one’s breath. It extends the image of the smile but subverts its positive connotation with a disgruntled or repressed “mouthing off.” 
 Myriad subtleties  refers to the deft wordplay and behavior blacks resorted to in order to avoid offending whites and provoking retaliation. Under oppression, people create a language of their own, a medium through which they can communicate without interference from mainstream white culture. In a way, the linguistic subtleties mentioned here are their own kind of mask.
We Wear the Mask