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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.
by secretyellowjedi May 08, 2011