...an old toothbrush.是什么意思？

...an old toothbrush.

一把旧牙刷

an

art. 一（在元音字母前代替不定代词a），一任一某一，一个；

例句
He started eating an apple...
他开始吃苹果。

old

adj. 老的，古老的，以前的，（用于指称被替代的东西）原来的；n. 古时；

例句
She was old enough to absent herself from the lunch table if she chose...
她长大了，可以自己做主不来吃午餐了。

toothbrush

n. 牙刷；

例句
As soon as the bristles on your toothbrush begin to wear, throw it out.
牙刷毛一开始磨损，就把它扔掉。