Supposedly, usr was supposed to be “user”[0] but was switched after the Unix guys needed more disk space and split the file system into the / and /usr mount points.

Then /home was made because /usr was filled up with other things.

[0]: Rob Landley of busybox fame recalled on the busybox mailing lists some ten years ago http://lists.busybox.net/pipermail/busybox/2010-December/074...