"antirent" is an old or dialect word meaning opposing the collection of outstanding rents in upstate new york during the early 19th century after the death of stephen van rensselaer iii. If your teen uses it, they have most likely picked it up from a book, video game, period drama, or a regional grandparent rather than a modern peer group. There is no cause for concern here; the word is largely out of contemporary teen usage. If it appears in a school text, it is being studied for vocabulary or historical context. You can treat it like any other archaic vocabulary item.