Donk is another word for penis, on swole meaning errect.
Hence meaning keeping ones penis erect.
From the song, "I'm in love with a stripper REMIX, featuring Akon, R. Kelly, Twista -DIRTY" At 3.45" R.Kelly say's, "Reeereeereee coming down the pole, ain't no secret why i'm here; cause you keep my donk on swole"