spruitmPeople say "im going to get something offline" because they are lazy and run words together in their head.
She is basically trying to say "I am going to get a shirt off the internet", but also thinking of "online", so it comes out as she said it.
off the internet + online = offline
Still though, the concept of "coming online" means that you're coming on to the line, since it's not coming on online or being off online. The idea is that when you are connected, you are "on" line and when you aren't you are "off" line. Whether or not the concept of offline developed after online was being widely used doesn't really matter, it's still the same idea of the internet actually just being a line and people deciding whether to be on or off it
"I think you have an extra chromosome lying around somewhere..."-KaDONK.
RIP Shane, you will be missed