Confused, Mixed
This was pure gold long ago, but now it's not. Someone got confused and mixed it with something else.
You could swear this thing was gold a long time ago. Now it's a mix of different things.
さく, しゃく
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It's mixed (and not pure anymore) because the confused person put it in that sack (さく) filled with various other things.
Since it was in that sack for so long, it just ended up getting mixed with the other stuff in that sack. It was gold a long time ago, but now it's mixed with everything else.