It’s an adverb that can be used in a number of different ways:
1 It can
mean particularly
savage and violent;
2 It can mean in a direct and uncompromising way, where no attempt is made to disguise unpleasantness;
3 It can mean to an extremely unpleasant or harsh degree;
1 He was brutally attacked in the
street;
2 It was a brutally
candid review of her performance;
3 The thaw followed weeks of brutally cold weather;