In Chapter 4, Par 1, the traveling Physician Vilbert employs hyperbole to excite and mock the young protagonist. He tells the child Jude that the university at Christminster is such a distinguished place that even the old women who do the laundry speak Latin:
‘You’d say so, my boy, if you’d seen it. Why, the very sons of the old women who do the washing of the college can talk in Latin – not good Latin, that I admit, as a critic: dog-Latin – cat-Latin, as we used to call it in my undergraduate days.’
‘Well – that’s more for the men who are in training for bishops, that they may be able to read the New Testament in the original.’
‘I want to learn Latin and Greek myself.’
‘A lofty desire. You must get a grammar of each tongue.’
‘I mean to go to Christminster some day.’
Hardy’s Victorian readers would have known that "cat-Latin" and "dog-Latin" aren’t real languages—they're playground languages more contemporarily known as “pig-Latin.” One “speaks” it by moving the first letter of a word to the end and adding the syllable “ay”. For example, “Jude Fawley” would be “Udejay Awleyfay.” It is certainly possible that old laundrywomen in Christminster would have known about this game, but it’s definitely not a classical language like the ones the men “training for bishops” speak and read.
Vilbert is making fun of Jude here, assuming correctly that the child won’t understand that cat-Latin isn’t a real language. He’s right, and Jude listens wide-eyed to these tall tales. Stories like this contribute to Jude’s romantic imaginings of what life in Christminster would be like, especially compared to the parochial environment of Marygreen. They also contribute to his feelings of disillusionment and foolishness when he later discovers what life there is actually like.