Last chance undies is your last pair of clean underwear1, reminding you that this is your last chance to wear any unless you get your laundry done.
应对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假如你不想裸奔的话,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,我已经把我的应对内裤穿上了。