One way to hold this word: a person bent over the thing they care about, lamp on, door shut, while the noise of everyone else goes on somewhere behind them. All the meanings sit somewhere on that line — some of them looking at what is on the desk, some of them looking at the closed door.
From the word’s history
The origin is not settled. The word is recorded from 1951 in US student slang. A capitalised Nerd appears a year earlier in Dr. Seuss's If I Ran the Zoo as the name of an invented animal, and two other guesses are offered: an alteration of nerts (crazy), or hearing he's inert as he's a nerd. All of these are proposals, not a proven line to the modern meaning.
來源沒有定論。可以確定的是一九五一年美國學生之間已經在用。前一年 Dr. Seuss 的童書裡出現過 Nerd，是一隻想像動物的名字。另外兩個猜測是：來自 nerts（瘋瘋的），或是把 he's inert 聽成 he's a nerd。這些都只是推測，不能當成這個字今天意思的由來。
Wiktionary, "nerd"
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