Literally, 伤不起 means "I can bear or accept it", while in Chinese, it is mostly used as modal particle in oral language. As a Verbal slang appeared these years from Internet, it is better to used this phrase at the Non-formal occasions.

Basically it means that someone cannot afford getting hurt. For example:我们都伤不起啊！We cannot afford getting hurt. :)

只是一句网络用语，用不了多久也许就没什么人用了。

伤不起，表示自己很脆弱。 就像你如果没钱，你会说“太贵了，买不起。“ 例句：我太脆弱了，伤不起。／房价太贵了，伤不起！／考试成绩出来了，看到结果真是伤不起～

It's a slang word that is typically used online. It literally means that someone can't bear the hurt/pain (not physical, but mental). Actually it's typically used kind of humorously.