mesg [display the current terminal writability state]
mesg yes [make your terminal world-writeable]
mesg no [make your terminal not world-writeable]

This command simulates the effect of the unix mesg(1) command. When you specify the 'yes' flag, other users on your system may write messages directly to your terminal, which will mess up your ircII screen. If you specify the 'no' flag, other users will be prohibited from writing to your terminal. If you do not specify an argument, the current value is used.

This operation is permanent and transcends EPIC; when you exit EPIC the value you set here will persist until you log out.

Back in the day when dinosaurs roamed the earth and everybody on the internet was using a unix shell account, the 'talk' program was very popular and was used to talk to other users on other machines who were not using irc. Eventually some yutz on irc figured out that you could corrupt a victim's terminal emulator making it impossible to see anything by forging talk requests and inserting specially crafted data into it.

If your unix host runs the talk daemon, you should /MESG no to avoid being the victim of a “talk bomb”. You really should turn off the talk daemon since “talk bombs” are about the only thing they're used for these days. If your unix host is not running 'talk' then it really doesn't matter what your MESG setting is.

Because write-access control to your terminal is controlled entirely through ordinary unix file permissions, users that are not subject to ordinary unix file permissions (ie, root) can never be prohibited from writing to your terminal. This is not a bug in epic, this is how unix works.