风吹裤衩蛋蛋凉
[fēng chuī kù chà dàn dàn liáng]
Translating directly as 'The wind blowing against undergarments makes the testicles feel cool', it's a colloquial way of expression. In modern slang used humorously or playfully, it can refer to someone being caught unawares or inappropriately exposed.