心裡的那顆小石頭
這一組是石頭在心裡。事情小到你說不清楚，可是它一直回來。
verb · C1
If something niggles you, it keeps bothering you in a small way: it is not big enough to be a real problem, but you cannot put it out of your mind and it keeps coming back.
隱隱困擾
核心畫面
You can think of niggle as something very small that keeps rubbing — a stone in your shoe rather than a blow. It is too small to stop you, and too constant to forget. Every use keeps both halves of that picture: the size is tiny, and it will not leave you alone. The stone can sit in your mind, in your mouth as a small complaint, or in your knee.
可以想成鞋子裡有一顆小石頭。它不會讓你走不動。可是每走一步，它就提醒你一次。niggle 的每個用法都保留這兩件事：…更多收起
可以想成鞋子裡有一顆小石頭。它不會讓你走不動。可是每走一步，它就提醒你一次。niggle 的每個用法都保留這兩件事：份量很小，而且一直在。那顆小石頭可以在心裡（一絲懷疑、一件放不下的小事），可以在嘴上（一句小抱怨、為細節爭不停），也可以在身上（膝蓋那邊的一點痛），甚至在東西上（新機器還沒磨順的小毛病）。From the word’s history
Wiktionary records niggle from 1599, origin uncertain, likely borrowed from dialectal Norwegian nigla (to be stingy, to busy oneself with trifles), ultimately from Old Norse hnøggr (stingy), related to Old English hnēaw. If that route is right, the word has been handling small amounts from the start.
Wiktionary 說 niggle 1599 年就有紀錄，來源不確定，可能借自挪威方言 nigla（小氣、為瑣事忙來忙去），再往上連到古北歐語 hnøggr（吝嗇），跟古英語 hnēaw 同源。如果這條線成立，它從一開始碰的就是「小」——小氣、小事、小份量。這一點正好和今天的每個用法接得上。
Wiktionary etymology text supplied in packet (kaikki:3f6f69a190036ae4:621244510:3152 and :621247662:8293)
Three branches
挑小毛病、小抱怨
這一組是石頭被講出來。方向剛好反過來：不是事情磨你，是你在磨那個小細節。
verb · C1
To keep arguing or complaining about small details that do not really matter.
挑小毛病
身上和東西上的小狀況
這一組是石頭從心裡移到身上、或移到東西上。份量的邏輯完全一樣：會提醒你，但不會擋住你。
noun · C1
A small pain in part of your body that does not stop you doing things, but that you keep noticing.
小傷、隱隱作痛