Last chance undies 应急内衣

Last chance undies is your last pair of clean underwear, reminding you that this is your last chance to wear any unless you get your laundry done. (Source: urbandictionary.com)

应急内衣是指你的最后一套干净的内衣，穿上这套内衣就意味着“如果你不想裸奔的话，你得赶快洗衣服了”。

For example:

Dude, I gotta do laundry today. I've got my last chance undies on!

哥们，我今天得洗衣服了，我已经把我的应急内裤穿上了。