In fact the English is clearly taken from the Greek and constitutes one of the definitions for the word.
As with the English it actually defines an individual whose views or behavior are slightly out of tune with our own. Those with views or behavior totally out of tune with our own are generally defined in common usage as mad (bellos).
The other use is more complex and requires an understanding of the motivation of the speaker, specifically it describes someone who disagrees with you for any reason but with whom one is not in a position to argue.
For example, the owner of a much cherished mint condition Singer Gazelle (1499 cc petrol engine, four speed 1969 vintage) once told his mechanic that it was time to service his (the owner's) car.
"When you change the oil in the gearbox", the owner explained with care, "you must use normal engine oil and NOT the thicker gearbox oil, as this is a peculiarity of design with that make of car"
Some gear crashing days post service, the owner returned to the garage mystified at the curious grinding noises coming from under the floor of his once smooth running car. The mechanic pointed out that the car is very old and problems must be expected, which was why he had considerately filled the gearbox with extra heavy-duty gear oil to smooth things out.
Once the shouting dies down the mechanic will perhaps change the oil again if threatened with legal action or else a large spanner or otherwise if he is paid extra, but he will later dismiss this troublemaking client to his neighbours as an ekkendrikos.