1244 Fistful

Someone told me not long ago that he was making fistfuls of money. So why do we say fistfuls, when we say sons-in-law instead of son-in-laws and attorneys general rather than attorney generals?
Shouldn’t we be saying fists-ful of money? Or spoons-ful of medicines? Spoonsful of sugar make medicines go down?
The fist is being pluralized, not the fulls. It’s not a fist that is fulls of money. There are numerous fists that are full of money. Or spoons. Or hands. He had his hands full of handsful of money.
And while we’re at it, what does “making money hand over fist” mean? I understand physical references like “he had his hands full.” Or “he didn’t have a leg to stand on.” Or “he was in over his head.” And when it comes to money, I can picture the person who is “laughing all the way to the bank.”
But I have a hard time figuring out the phrase “making money hand over fist.” I can see a guy climbing a rope hand over hand. Or someone giving someone a hand up and not a hand out. But why hand over fist?
I put my hand over my fist and it doesn’t look like anything that makes money. Unless I’m making money for “scariest shadow” in a game of shadow puppets and my puppet looks like a grenade.
Wait a minute, it also looks like the result of a game of rock-paper-scissors. The paper result. Paper covers rock. The “fist” being the rock, the “hand over” the fist being the paper.
So maybe back in the old days, when they made up sayings, people played really high stakes gambling games of rock-paper-scissors.
And choosing paper made fistsful of money...
America, ya gotta love it.