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.
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' etymology supplied in evidence packet (kaikki:3f6f69a190036ae4:410212421:35166)
Two branches
全副心思放在一個主題上
這一族看的是桌上那樣東西。重點在「鑽得多深」，而不在他跟人群的距離。
noun · B2
Someone whose interest goes almost completely into computers, machines, or other technical things, so other people feel that nothing else matters to them. Saying it about another person is usually a small put-down.
電腦怪咖
因此被放在人群外面
這一族看的是那扇關起來的門。說話的人不管他桌上是什麼，只注意到他不在人群裡。
noun · B2
Someone who is clever and serious about study or thinking, and who stays quiet and away from big social groups. It is slang, and it can sound friendly or unkind depending on who says it.
書呆子