The couch crasher develops a sofa circuit of sympathetic or co-dependent friends' places with sofas to crash on. Only losers do the sofa circuit, because they are perpetually broke and too lazy to get a regular job and live a responsible life.
Tim has been on the sofa circuit for a year, crashing at different friends' places till they get sick of him and kick him out, then he moves on to the next friend's sofa.
The pain you get in your ass from sitting on it all day. Discomfort can also be in the lower back.
This was referenced in S06 E21 of the popular TV Show "New Girl" (2011-2018) when Jessica Day, suffering from heartbreak, refused to leave the house.
"Let's just get you out of this house"
"Yeah, right"
"Well what are you going to do? You're gonna sit on this couch all day long? You'll get sofa butt. C'mon, let's go get some ice cream."