yī yán nán jìn

一言难尽 (一言難盡)

cannot fully express in one sentence

it’s a long story; it’s hard to explain in a few words

Example:

其中细情, 一言难尽。
Qízhōng xì qíng, yīyánnánjìn.
It would take a whole book to give all the details. or It’s impossible to describe in a few words.

说来话长, 其中底细, 一言难尽。
Shuōlái huà cháng, qízhōng dǐxi, yīyánnánjìn.
It would take too long to tell you that in full.