可想而知 ke3 xiang3 er2 zhi1
1 You can say 他不认真工作，结果就可想而知了。
That means “He didn’t work hard, and the result was obvious.”
2 You can say 他不好好学习，考试的结果就可想而知了。
That means “He didn’t study hard, and the test result was obvious.”
So this phrase actually means obvious(for a fact), or can be assumed with little thought.