A line was going straight, and then it did not. Something is moving forward — a bike, a ball, a person keeping a promise — and suddenly it goes sideways out of that line. Two things travel with the picture everywhere on this page: the movement was already under way, and the sideways part is abrupt rather than gradual. What changes between the senses is only what the straight line was made of, and whether anything chose to leave it.
From the word’s history
The source traces the word back to Old English sweorfan, whose meanings it lists as to file, rub, polish, scour, turn aside. That last one, turn aside, is the thread the modern word holds on to. The source does not explain how the rubbing meanings and the turning meaning relate, so nothing more is claimed here.
資料把這個字一路記到古英文 sweorfan，並列出它的意思：磨、擦、拋光、刷，以及「轉向一邊」。今天的 swerve 抓住的是最後那一個。至於磨擦那幾個意思跟轉向怎麼連起來，資料沒有說，所以這裡就不多推。
packet Wiktionary etymology_text for swerve: 'from Old English sweorfan (to file; rub; polish; scour; turn aside)'; kaikki:3f6f69a190036ae4
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