When you're simply too lazy or poor to get more tissue so you pick up some old crusty tissue out the bin to blow your nose on so you don't have to move or buy tissue.
"Hey man you still got that cold?"
"Yeah, I was so tired I had to Crusty Blowback I couldn't be bothered to get more tissue "
OR
"Fuck man I just had a wank and I had nothing to clean with so I had to Crusty Blowback!"