他们就是西山背后的印第安人，一谈到这个题目他就滔滔不绝说个没完。

For it was his contention that they were the Indians beyond the Western Mountains, and on this subject he would talk endlessly.

youdao

他一开口就滔滔不绝。

When he started to speak, the words just spilled out.

《牛津词典》