This phrase is commonly used when someone is pulling a face that you would usually associate with taking a poo (to strain) also often associated with a cum face.
Will you please stop giving me the poo face, i know you're stressed - calm down!
Poo Faced girls who like to spend hours on the sunbed and end up having a face that resembles a large poo like structure, large "dow" like faces or fat chops are very common in the poo face race.