Praise be to Allah.
If that is done by means of plucking, then it is haraam, and indeed it is one of the major sins, because it is the “namas” for which the Messenger (peace and blessings of Allaah be upon him) cursed the one who does it. But if it is done by cutting or shaving, then it was regarded as makrooh by some scholars and forbidden by others who regarded it as a kind of “namas”. They said: “ ‘Namas’ does not only mean plucking, rather it is a general word which includes all ways of changing the hair for which Allaah has not granted permission, if it is on the face.” But what we think is that women should not do that unless the hair of the eyebrows is so thick that it hangs down over the eyes and affects her vision, in which case it is o.k. to remove that which is causing the problem.