Think of the moment a plane tips its nose down and drops towards the ground. Not a slow, gentle descent, but head-first, steep and fast, the kind of fall that makes people catch their breath. Every use of nosedive can be held inside that one picture. What is dropping may be a plane, a person tripping forward, or a line on a sales chart, but the shape of the fall stays the same: nose first, straight down, and quicker than anyone expected.
From the word’s history
Wiktionary records the word simply as 'nose + dive'. The two parts still say the whole picture: the nose goes first, and the movement is a dive — downward, steep and fast.
Wiktionary 記載 nosedive 由 nose 加 dive 組成。兩個字合起來就把畫面講完了：鼻子（機頭）在前，動作是 dive——往下、又快又陡。後來用在價格和狀況上的意思，借的就是這條下墜線。
Wiktionary entry 'nosedive' (packet source_ref kaikki:3f6f69a190036ae4:1215680536:8801): 'From nose + dive.'
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