Surely I heard that wrong.

[darxus]

"Do you have some change so I can eat something?"
"No."
"I have a fin."

Comments

[beowabbit]

I thought I remembered that that was a slang term for money, and Wiktionary confirms it:

Noun
fin (plural fins)
9. (US, slang) A five-dollar bill.

So my guess would be s/he was trying to change a 5 so s/he could use the vending machines.

[darxus.livejournal.com]

We were in the Kendal Square food court.

[woodwardiocom.livejournal.com]

Assuming he's not using the slang for money, I would assume he was playing up some sort of birth defect for sympathy.