Either way this is unacceptable (3,2)
I believe the answer is:
not on
I believe this is a double definition.
'either way this is' is the first definition.
'unacceptable' is the second definition.
(something 'not on' is unacceptable or frowned upon)
(Other definitions for not on that I've seen before include "Improper - impossible" , "Modern usage for unacceptable" , "Informally, cannot be permitted or tolerated" , "Out of the question" , "Definitely inadvisable" .)