古体的，已不通用的曾经通用而现在不再使用主要指早期风格或时代的单词或语言的，与之有关的或具有其特征的。

Of, relating to, or characteristic of words and language that were once common but are now used chiefly to suggest an earlier style or period.

youdao

thou art是you are的古体。

'Thou art' is an archaic form of 'you are'.

《牛津词典》

我瞧着表格上难以辨认的斯拉夫古体字，明白自己已经走出了处处让你舒适和给你方便的地区了。

As I stared at the incomprehensible Cyrillic script, I knew that I was way out of my comfort zone.

youdao