The Oxford English Dictionary indicates the word can be used as an adjective, adverb, or noun. Shakespeare uses it as a noun, with the meaning that another responder has already provided. The OED also says that the word, in Shakespeare’s time, “was a more dignified word than now.” Beyond that, in the context of the play, hurley-burley, with the meaning of battle, suggests that someone will win and someone will lose (because such are the dynamics of warfare). The words immediately introduce verbal irony because the day will signal a great loss and a great victory (thus, as elsewhere in the play, the witches equivocate), and also situational irony, for both the victory and defeat are of an unforeseen and surprising nature. It can also be noted that the word “hurley-burley” provides a good example of onomatopoeia, a word which suggests its meaning.