verboten
(Strictly) forbidden or prohibited.
Before that, students and professors could date whomever we wanted; the next day we were off-limits to one another—verboten, traife, dangerous (and perhaps, therefore, all the more alluring).
Where it comes from
Unadapted borrowing from German verboten (“banned, forbidden, prohibited”). Doublet of forbidden.
Adapted from the Wiktionary entry verboten (CC BY-SA 4.0).