Think of someone tied to one thing so tightly that the thing decides the shape of the day: when they get up, where they go, what they think about at lunch. The word names the person by that tie, not by anything else about them. That is why it is heavy when the tie is a drug, and why it becomes a joke when the tie is bicycles or coffee — the picture is the same size, and only the thing on the other end of the rope has changed.
From junk in the sense of a narcotic drug, plus the ending -ie, which makes a word for a person tied to the thing named.
資料記載這個字是 junk（毒品）加上字尾 -ie。-ie 在英文裡把一樣東西變成「跟它綁在一起的人」。所以這個字從字面上就是「被那樣東西綁住的人」，這也說明了為什麼它一開口就這麼重。
Wiktionary entry for junkie (etymology_text supplied in the evidence packet, kaikki:3f6f69a190036ae4:781338311)
Two branches
被毒品綁住的人
這是繩子另一頭綁著毒品的情況，也是這個字原本的意思和全部力道的來源。
noun · B2
Someone who takes a dangerous drug and cannot stop, especially heroin. The word is slang and it is recorded as derogatory: it names the whole person by the addiction, so people who want to speak carefully say someone with a drug addiction instead.
毒品成癮者
離不開某樣東西的人
同一條繩子，另一頭換成無害的東西。正因為原本的畫面那麼重，拿來講咖啡或球賽才會好笑。
noun · B2
Put after another word, junkie means someone who loves that thing so much that they can hardly go without it — a travel junkie, a sports junkie, a technology junkie. It is used humorously.
……迷、離不開……的人