I don't know
The finger's shape is secret and unknown. It's the same as "God doesn't know, ghost doesn't know".
Idiom explanation
[explanation] the finger is secret and unknown. It's the same as "God doesn't know, ghost doesn't know".
Idioms and allusions
[source]: Zhang Yi's work in the enemy area: "besides, we came in without knowing it."
Discrimination of words
It can be used as an adverbial or a clause to describe a secret thing
I don't know
pierce a willow leaf with an arrow from the distance of a hundred paces - bǎi bù chuān yáng
true gold does not fear fire - zhēn jīn bù pà huǒ liàn
extraordinary as if done by the spirits - guǐ fǔ shén gōng