... is used in the Everything2 Chatterbox in place of a space.

When addressing (with /msg or /tell) someone with a space in their name, say, Joe Blow, one would type /msg Joe_Blow or /tell Joe_Blow. see also E2 FAQ: Chatterbox

This is a service of the Chatterbox Helpdesk to people who don't feel like, well, doing what E2 is about, namely, reading the manual. Anytime anyone asks, "Why can't I send a message to Joe Blow?" we now have the option to reply with _. Putting brackets around the underscore and directing them to "poke at the line" might help, too.