2. Slang. (negative) - to A.)acknowledge and accept partial responsibility for the presence/actions of another person and the effects thereof, or B.)to deny all responisbility and/or attachment to said individual and/or their actions and any effects thereof.
"Who's the dude you came in with?"
"Oh, that's Keebler. Don't worry, man. I co-sign for him. He's cool."
2. (negative scenario - some random party)
"I think your buddy's about to get his ass beat."
"Who said he's my buddy? I just walked in next to him, I didn't co-sign for him."