irssi/docs/help/in/query.in

@SYNTAX:query@

Starts a private conversation with the nick.  All text you
type that would normally be sent to your channel now goes to
the specified nick in the form of MSGs. 

Usually this command opens up a new window, too.

Note: It removes the active query from the active window when
used without parameters, like UNQUERY.

See also: WINDOW, MSG, SET QUERY, UNQUERY