It’s partially a reference to the fact that Southwest DC used to be swampland. The area west of the Washington Monument and south of Constitution Ave (the reflecting pool, Lincoln Memorial, Korean War Memorial, Vietnam Veterans Memorial, Constitution Gardens, MLK Memorial, FDR Memorial) was all infill land as part of the McMillan plan. Take a look at the original L’Efant plan for the city, and note that the modern day site of the Washington Monument used to have the Potomac right on it:
Also note that the peninsula that now has the Jefferson Memorial did not exist then either. In fact, the Lincoln Memorial was built before the reflecting pool, and had swampland between it and the rest of the mall for a time. This was along the way of executing the McMillan plan of 1902, which updated and expanded L’Enfant’s original plan.