A prejudgment is an opinion you have already formed, before you have looked at all the facts. Like the verb prejudge, it is not a compliment — it says the opinion came too soon, and we use it with disapproval. You will meet it in careful English and in the law. Though actually, lawyers also use prejudgment in a drier, technical way, for things a court does before the final decision (for example prejudgment interest). That is a different path. The sense here is the everyday one: making your mind up too early, and being blamed for it.