This is the 8267th most frequent Russian word.
маском
"Маском" is the instrumental case of "маска," meaning "mask" in English.
Here, 'маском' refers to the instrument or the equipment, implying 'with a mask'.
Я смотрел фильм, снятый маском.
I watched a movie filmed with a mask.
In this sentence, 'маском' denotes an abstract metaphorical usage of 'mask' representing a covering or disguise.
Он скрывал свои эмоции под маском веселья.
He hid his emotions behind a mask of cheerfulness.
The word 'маском' here specifies an actual mask worn as part of attire, such as for a costume event.