One way to hold this word: a line kept clean all the way back. Nothing was added from outside, generation after generation, and what comes out at the end is an animal built for one thing — speed — and built for it. The word carries both halves of that picture: where the thing came from, and how easy it makes the hard thing look. Move the picture onto a person and only the second half is really meant.
The Wiktionary entry supplied in this packet records the word simply as thorough + bred — bred all the way through.
本批資料的 Wiktionary 條目只寫了一句：thoroughbred 來自 thorough 加 bred。thorough 是「徹底、從頭到尾」，bred 是「被養育出來的」，合起來正好就是核心畫面。
Wiktionary entry supplied in evidence packet (kaikki:3f6f69a190036ae4:1128414636): `From thorough + bred.`
Two branches
純種的動物
字面的一支：血統線本身，名詞講那匹馬，形容詞講那條線。
noun · B2
A horse whose parents both come from the same fine breed, with nothing mixed in. Written with a capital T, a Thoroughbred is one particular racing breed whose family history is kept in an official book.
純種馬（尤指純種賽馬）
天生就該做這件事的人
把那匹馬的形象移到人身上：留下的是「跑得快又不費力」，血統那一半只剩比喻。這是一種比喻上的連結，不是說這個意思在歷史上一定這樣長出來。
noun · C1
Someone who seems born to do something and does it at a very high level with no visible effort. It is praise, and it is heard more in British English.
天生好手