I thought it was a reference to the Mac OS X `~/Public/Drop Box` directory, which was a write-only place for people to send files to your user, which has been around since the first OS X beta came out in 2000.
I vaguely remember being told to put assignments in a drop box (like a mail box on campus) in the mid 2000s at least, and I'm sure it wasn't a new concept then.
Oh dang, you're right. Mac OS X was my first Mac OS, but it looks like the Drop Box concept existed long before OS X. Here's a reference from 1991 titled "AppleShare Drop Box: Access for System 6 and 7 Clients" https://www.savagetaylor.com/TIL/TIL09033.pdf
