From BiblePortal Wikipedia

Webster's Dictionary [1]

(n. & a.) See Woe.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [2]

(usually אוֹי or הוֹי , Οὐαί , all onomatopoetic) is often used in the English version where a softer expression would be at least equally proper. "Wo to such an one!" is in our language a threat, or imprecation, which comprises a wish for some calamity, natural or judicial, to befall a person; but this is not always the meaning of the word in Scripture. We have the expression "Wo is me," that is, Alas, for my sufferings! and "Wo to the women with child, and those who give suck," etc., that is, Alas, for their redoubled sufferings, in time, of distress It is also more agreeable to the gentle character of the compassionate Jesus to consider him as lamenting the sufferings of any, whether person or city, than as imprecating, or even as denouncing them, since his character of judge formed no part of his mission. If, then, we should read, "Alas, for thee, Chorazin I alas, for thee, Bethsaida!" we should do no injustice to the general sentiments of the place or to the character of the person speaking. This, however, is not the sense in which wo is always to be taken, as when we read, "Wo to those who build houses by unrighteousness, and cities by blood;" wo to those who are "rebellious against God," etc., in numerous passages, especially of the Old Test. The import of this word, then, is in some degree qualified by the application of it; where it is directed against transgression, crime, or any enormity, it may be taken as a threatening, a malediction; but in the words of our Lord, and where the subject is suffering under misfortunes, though not extremely wicked, a kind of lamentatory application of it would seem to be most proper. (See Imprecation).