手指最前面那一截
字面上的那個部位，以及直接用那個部位做出來的動作。
noun · B1
The end part of your finger, furthest from your hand — the soft part you touch, feel, and press things with.
指尖
核心畫面
Hold up one hand and look at the last small piece of a finger. That piece reaches first, touches first, and feels most. It is also the point of you: past it there is only air. One way to hold fingertip is as that reaching, feeling far end. Everything else the word does comes from one of those two facts. Because it reaches first, whatever is already under it needs no searching. Because it is the point of a person, going out to it means going all the way. And because it touches so lightly, a touch with it is the smallest push there is.
把手舉起來，看著手指最前面那一小截。那一截最先碰到東西，也最有感覺。它同時是你身體最外面的一點：再過去就是空氣了。f…更多收起
把手舉起來，看著手指最前面那一小截。那一截最先碰到東西，也最有感覺。它同時是你身體最外面的一點：再過去就是空氣了。fingertip 可以想成那個「先到、能感覺」的末端。這個字其他的用法，都是從這兩件事長出來的。因為它最先到，所以已經在它底下的東西不必去找。因為它是人最外面的一點，所以「一路到那裡」就是「整個人都是」。又因為它碰得很輕，用它碰一下就是最小的一推。From the word’s history
A compound of finger and tip, the same way English makes tiptoe and toetip.
由 finger（手指）＋ tip（尖端）合成，跟 tiptoe（腳尖）、toetip 是同一種造字方式，字面意思就是今天的核心畫面。
Wiktionary entry supplied in packet (kaikki:3f6f69a190036ae4:714306488), etymology_text: From finger + tip. Compare tiptoe and toetip.
Three branches
在指尖上＝不用找就拿得到
指尖最先碰到東西；如果一樣東西已經在指尖底下，那就表示不必去找它。
noun · B2
To have knowledge or information ready and close by, so that you can use it straight away without having to go and look for it.
隨手可得
一路到指尖＝整個人都是
指尖是人身上最外面的一點，所以「延伸到那裡」就等於「沒有一處不是」。
noun · C1
Completely, in every part: used after a word describing what someone is, to say they are that thing all the way through.
徹頭徹尾