Manipulate, Chastity
If someone has their fingers in your syrup, it's just another way of saying they manipulated you. Cause only slimy people manipulate other people, so their fingers are slimy and sticky because they're covered in your syrup from mucking up your life. This also has another meaning: chastity. Just imagine if someone wanted you to keep your chastity and they called s.e.x. when someone "puts their fingers in your syrup." That would be pretty effective, right? Cause it's like, super gross.
"Ah golly gee wiz, he sure got his fingers all up in my syrup again, dagnabbit." You know, this saying we say all the time when someone manipulates us into doing something.
そう
あやつ, みさお
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If you want to manipulate someone else it's going to have a permanent effect on your soul (そう). Being manipulative isn't cool. And your soul will suffer forever for it.
What happens to your soul, you ask? It gets covered in syrup. That's probably where the syrup fingers phrase came from in the first place.