Take a tape measure and run it once round the middle of something, at the place where it pulls in. That single line is the whole word. Round a person it gives a number a tailor or a doctor down. On a dress it is a line the designer can move up or down. Down the side of a car it is the line under the windows. Same line, three surfaces.
From the word’s history
The supplied Wiktionary entry is explicit: waistline is waist + line. The compound is transparent, and every use keeps both halves — a waist-height line drawn once round a body, a garment or a vehicle.
packet 附的 Wiktionary 資料寫得很直接：waistline 就是 waist（腰）加 line（線）。這個複合字是透明的，拆開就看得懂。所有用法都同時保留這兩半：在腰的高度上，繞一圈畫出的一條線。知道這一點，衣服和車子的用法就不必另外背。
Wiktionary entry supplied in this batch's evidence packet (source_ref kaikki:3f6f69a190036ae4:832032578, From waist + line)
登入就可以看每一支怎麼連回核心畫面。