OK, so now I know why tweening's called that, and I feel like the biggest idiot in the world :-/
Edit: corrected. But I can't delete? Oh, I have replies :)
Tweening actually goes back to traditional animation where the keyframes (another borrowed term) were hand-drawn by senior artists and the frames in between were added by junior artists. This process was called inbetweening, or tweening for short.