Sloshing the knob is a phrase for a blowjob.
Sloshing is the noise of water or liquid (an onomatopoeia).
Knob is slang for a penis.
Hence, sloshing the knob constructs the imagery of a blowjob.
Variations include sloshing the knob, sloshing my knob, would you like your knob sloshed, and so forth.
Daniel: Hey, want to slosh my knob?
Alyssa: Not now, I have morning breath.