The name isn't ours
A Swede gave it to us, reluctantly.
The sauce was outrageous, the spoon was nowhere, and one of us went in with a finger. Our server, Swedish, nodded like he'd seen it a thousand times. "Ah. We have a word for that."
We asked what it was in Swedish. He thought about it, smiled, and said, in the thickest possible accent: "Bonus Finger."
That's the whole etymology. We just decided it deserved a tool.