This is one of the many, many variations on what is called, in my neck of the woods, emote syntax. The version you describe may be a descendant of "emotes" (generally meaning a freeform message display) as they are typically implemented on MUDs, MUSHes, MUCKs, and so on, which uses a command form like:
:jumps for joy
which results in everybody in the same virtual "room" seeing, for a user named Somebody,
Somebody jumps for joy.
In IM, I personally use
*action* emote syntax.
<action> is also popular, and
/action has made some headway from the MMORPGs where it's used for "social actions". People routinely just invent their own personal syntaxes out of whole cloth, which is another likely origin for your
The general idea of emote syntax may go back to the first MUD, which opened in 1978, and certainly isn't much older than Usenet, which dates to 1980.