Aufs
From MEPIS Documentation Wiki
One of the cheatcodes or boot options. It adds a read/write filesystem module that allows software to be installed and used during a LiveCD session, but the installation does not persist beyond the session.
MEPIS 8.5
In MEPIS 8.5, there is a boot option on the main GRUB menu called "R/W filesystem." Select that when you boot with the LiveCD if you wish to temporarily install software.