Meaning:

“迷” means “lost,” and is pronounced the same as “谜,” which means “mystery,” “之” is an indicator of a modifier, and “自信” means “self confidence.” This term coined by Chinese netizens is often used derogatorily to refer to those who are arrogantly and stubbornly assured of their judgement or decision, so much so that others think they are “lost” in their confidence or they have drawn their confidence from a “mystic” origin.

Example:

A: 小花要和她男朋友分手，可是对方怎么也不相信她是真心要分手。

Xiǎohuā yào hé tā nánpéngyou fēnshǒu，kěshì duìfāng zěnme yě bù xiàngxìn tā shì zhēnxīn yào fēnshǒu。

Xiaohua wants to break up from her boyfriend, but he just cannot believe that she means it.

B: 有些男生就是迷之自信，总觉得女生闹分手就是撒娇求关注。

Yǒuxiē nánshēng jiùshì mízhī zìxìn，zǒng juédé nǚshēng nào fēnshǒu jiùshì sājiāo qiúguānzhù。

Some men are often over confident. They think women talk about breaking up just as a means to get more attention.