Her anger and resentment were hidden behind a 'mask' of friendship. [=she pretended to be friendly but was really angry and resentful]
('American football') The lineman grabbed the running back's 'face mask'.
Doctors wear surgical 'masks' in the operating room.
(..) A lot of people think it's an igloo, but it's really just a plastic divider. Can you go in the bubble? Well, you have to put so many things on because of the germs. The gloves, the mask, it's a whole production. So then he makes his own bed? Well, that's one of the things we fight about. (..)1992 TvShow: Seinfeld Title: The Bubble Boy Season: 4 Episode 7