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;