https://ico.org.uk/media/action-weve-taken/decision-notices/2017/2013864/fer0646554.pdf
“The term ‘vexatious’ is not defined in the legislation. In Information Commissioner vs Devon County Council & Dransfield1, the Upper Tribunal took the view that the ordinary dictionary definition of the word vexatious is only of limited use, because the question of whether a request is vexatious ultimately depends upon the circumstances surrounding that request. The Tribunal concluded that ‘vexatious’ could be defined as the “…manifestly unjustified, inappropriate or improper use of a formal procedure” (paragraph 27). The decision clearly establishes that the concepts of ‘proportionality’ and ‘justification’ are central to any consideration of whether a request is vexatious.”
