At first I thought it was a tag, a signature maybe some random logo of a local graffiti artist or propaganda.
Adam explained to me that during WW11 when the Germans had occupied Poland an underground army formed to retake Poland. WP was used throughout the resistance to bring hope and was a symbol that strengthened the Polish people.
This was literally an underground army because they used the sewer systems to communicate and disrupt German occupations.
The other highly interesting thing Adam taught me, was that the Warsaw Uprising consisted of mainly youth (most likely between the ages of 11- 17). The ones that were too young to fight in the war or the ones left behind because they were not deemed as a threat to the German installations.