A very hackneyed phrase that means you live once and shouldnt go doing stupid things thinking your invincible. thats what it means to me. im not trying to be rude its just people do ridiculous things taking advantage of that phrase.
It means a free pass for morons to continue to do moronic things. It's the stupid version of carpe diem (seize the day) which told us to live each day to the fullest but it didn't excuse making an a** of yourself.