What "Can I sign you" means?

[RiTm]

Hi all!

There is such dialog in uncut version of "Friends" sitcom, season 1, episode 4:
(Ross pushes Joey in wheelchair)
Ross:Now you're gonna be in a wheelchair the rest of your life, pally-wally....
(now Ross notice another guy in wheelchair)
Ross:I am so very sorry. Can I ... sign you?
(tries to touch guy in the wheelchair, then moves away).

What does it means? It's some kinda of joke, but i can't find any suitable meaning of verb "sign" which would explain audience laughing in the background.

Who can help? Maybe it'll be more clear if you watch the video. Thanks a lot.

 

[Tullia]

I don't know if in other parts of the world it happens, but here in UK there's a bit of a joking custom that we "sign" plaster casts when people have broken arms or legs, leaving messages of goodwill and best wishes for the person's recovery. Normally, of course, you only sign the casts of people you know!

I would guess Ross says this here because he's in an awkward situation and can't think of anything else to say, and looking at the guy in the chair he is covered in casts, so he grabs at the first thing that comes into his head. It is a very silly thing to say in the situation, which is why it is funny.

 

[RiTm]

Tullia, thanks!

Though I found this meaning already. It's funny that this "signing" is popular and common in Belarus too, and all my friends knew it. But I didn't get it immediately for some reason .