John Brophy & Eric Partridge, "Songs and Slang of the British Soldier 1914-1918" (1931), referring to WW1:

"No less cheerful...is the following spirited ditty:

   Dan, Dan, the sanitary (or lavatory) man,
   Working underground all day
   Sweeping (or cleaning) up urinals,
   Picking out the finals,
   Whiling the happy hours away -
      Gorblimey!
   Doing his little bit,
   Shoveling up the ----,
   He is so blithe and gay,
   And the only music that he hears
   Is poo-poo-poo-poo-poo all day.

"Verses 7 and 8 ... had the variant:
   
   Always on the dap,
   Shoveling up the ----"


"On the dap" = dipping suddenly into water (I think).