D-notice means: A defence notice; an official request to news editors not to publish or broadcast items on specified subjects for reasons of national security (replaced in 1993 with the DA-notice).. The word is mostly archaic or historical; you will only encounter it in old texts, period drama, or school history rather than current teen conversation. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. As a place or proper-name reference it is purely descriptive.