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.
A term still used on Tahoe. Right click a file in Finder, Get Info. A file permissions option is "Write only (Drop Box)"
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.