! performs a boolean Not operation. Returns False if its parameter evaluates to True or True otherwise.

The parameter to this symbol will be cast to boolean automatically. Integer or decimal values equal to 0, empty strings '', the string 'false', and null are cast to False. All other values are cast to True.

[If: !(Right_Parameter)]
  ...
[/If]

<?LassoScript
  If: (Right_Parameter);
  ...
  /If;
?>