From world war two and used, usually by British soldiers, to describe the effect seen when the amunition propellant in a destroyed tank starts to ignite. The flames shoot out of any opening, most usually the turret hatches, extremely violently, like a Roman candle. This terrified tankers and as a sort of black humour they referred to it as brewing up.
Other nationalities sometimes referred to it as cooking off .. the ammunition was cooking off.
The tank was hit with an AP round and within seconds began to brew up.
To make a pot of tea. Usually used in the context of plural .. a pot of tea of several cups, rather than single cup, when you would instead say, "making a brew"
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"