One way to hold this word: a flat panel that sits just under the chin and lies across the chest. That patch of the body is the whole idea. Tie a cloth there and it catches the food a baby drops. it onto a pair of overalls and it becomes the square with a pocket on it. Pull a numbered square over your shirt at a race and it becomes the thing strangers read you by. Even an animal born with one white patch there gets described as wearing one. The material changes, the job changes, the place never does.
Wiktionary records that bib was first a verb meaning to drink heartily, from Middle English bibben, either from Latin bibo or simply imitating the sound. It then suggests, without certainty, that the clothing sense came either from wearing the cloth while drinking or from the cloth drinking up what was spilt.
維基詞典說 bib 最早是動詞，中古英語 bibben「大口喝」，可能來自拉丁文 bibō（喝），也可能只是模擬聲音。至於為什麼變成衣服，它用的是推測的語氣：可能因為喝東西時圍著它，也可能因為這塊布會把灑出來的東西「喝掉」。這條線索有趣，但來源自己也說不確定。
Wiktionary entry for bib (etymology 1), kaikki:3f6f69a190036ae4:218137725:52295 — "Originally verb sense 'drink heartily', from Middle English bibben... Noun sense (clothing) presumably either because worn while drinking, or because the clothing itself 'drinks up' spills."
Four branches
綁上去擋髒的那一塊
最原始的一支：胸前那塊布是另外綁上去的，任務是接住掉下來的東西。
noun · B1
A small piece of cloth or plastic that is tied under the chin and covers the chest, so that food does not fall onto the clothes below. Babies wear them most, and adults wear them for messy food.
圍兜
衣服上或衣服外面的那一片
同一塊位置，但不再是另外綁上去的布，而是衣服的一部分，或套在衣服外面的一層。
noun · B2
The flat part of an apron, a pair of dungarees, or a pair of overalls that comes up above the waist and covers your chest.
（圍裙、吊帶褲的）胸前那一片
動物天生的那一塊顏色
位置照舊，只是這一件沒有人綁，是動物身上長出來的。
noun · B2
A patch of a different colour on the front of an animal's throat and upper chest, in the place where a bib would sit.
（動物）胸前的一塊毛色
只活在固定說法裡
這一支不能拆開看：bib 在這裡只是舊衣物的名字之一，意思屬於整句話。
noun · C1
Your best clothes — the smart ones you put on for a wedding, a dinner, or a photograph. The expression sounds old-fashioned, and people use it as a light joke.
最體面的一套衣服