gut n. 内脏；肠子；胆量

a strong belief about someone or something that cannot completely be explained and does not have to be decided by reasoning:

对某人或某事的强烈信念，不能完全被解释，也不需要通过推理来决定

gut feeling作为“直觉”这个意思的使用可能是来源于人的感觉可能与肠子或内脏有关。

I have a gut feeling that the relationship won't last.

我有种直觉，这段关系不会太长久。