ircII help: set continued_line

Usage: SET CONTINUED_LINE <some text>
  This allows you to change the text which is displayed when a
  line is greater than than width of the screen.  By default,
  this text is a +, and each line greater than the screens width
  will have this prepended to all lines after the first.   You
  may use CONTINUED_LINE in conjunction with INDENT, or you may
  disable the CONTINUED_LINE character by setting it to <EMPTY>.

See Also:
  SET INDENT