composure, equanimity, self-control, self-possession, sangfroid, coolness, calm, calmness, tranquillity, good humour

informal cool

become very angry, fly into a rage, explode, blow up, erupt, lose control, go berserk, breathe fire, begin to rant and rave, flare up, boil over

informal go mad, go crazy, go wild, go bananas, have a fit, see red, fly off the handle, blow one's top, blow a fuse, blow a gasket, do one's nut, hit the roof, go through the roof, go up the wall, go off the deep end, lose one's cool, go ape, flip, flip one's lid, lose one's rag, lose it, freak out, be fit to be tied, be foaming at the mouth, burst a blood vessel, get one's dander up, go non-linear

British informal go spare, go crackers, throw a wobbly, get one's knickers in a twist

North American informal flip one's wig

Australian/New Zealand informal go crook

vulgar slang go apeshit