Am pretty sure it was Satoshi himself. There's a passage in Digital Gold by Nathaniel Popper where it's proposed in an exchange with that Finnish fella who helped him out with bitcoin.org at the start. I'll dig around.

Edit - It was an unknown stranger.


'While satoshi never discussed anything person in these emails, he would banter with Martti about little things.

In one email Satoshi pointed to a recent exchange on the Bitcoin email list in which a user referred to Bitcoin as a "cryptocurrency" referring to the cryptographic functions that made it run.

"Maybe it's a word we should use when describing Bitcoin. Do you like it?" Satoshi asked.

"It sounds good," Martti replied. "A peer to peer cryptocurrency could be the slogan."  '