Office chat, Friday.
"I bought a one piece yesterday!"
American colleague: swimsuit. British colleague: a boiler suit? Anime fan: a new volume. Speaker: a knee-length navy dress, on sale, very pleased with it.
From "one-piece dress", trimmed the way Japanese trims everything — this is the same machine that turned personal computer into パソコン, sexual harassment into セクハラ, and smartphone into スマホ.
It sits in a whole wardrobe of loanwords that English speakers can nearly follow: トレーナー (sweatshirt, from "trainer"), Yシャツ (dress shirt, from "white shirt" misheard), ジーパン (jeans, from "jean pants"), チャック (zip).
And yes: the manga is also called ワンピース, and it is the best-selling comic in history, which means the word does double duty in a way that no Japanese person finds confusing and every foreigner does at least once.
The rule underneath is simple and relentless: Japanese shortens a borrowed phrase to four syllables and keeps the half that carries the picture. One-piece dress → ワンピース. Convenience store → コンビニ. Once you can hear that rule, you can decode dozens of these on sight, which is the moment Japanglish stops being a joke and starts being a skill.