An outer appearance, way of acting, or a choice of words in a certain situation that "masks" your genuine feelings to make you more of a "bro".
Dan: Yo man, I totally failed that math test and feel horrible,,
Bob: really? (back of his mind: "yo you deserved it...)
*bromask on* dang i totally feel ya bro, study up and ace dat next one bro
Dan: Aight yee i guess so thanks man
Bob: Gotchu bro *bromask off*