Possible Clues:

• Sit (down) heavily
• Plunk down
• Sit ungracefully (with "down")
• Splat's cousin
• Sound of Alka-Seltzer hitting water
• Fall into a recliner
• Sit heavily, with 'down'
• Sit (down) unceremoniously
• Rock-in-pond sound
• Alka-Seltzer-into-water sound