I have not found a source where Thomas Zink explains the name, however <A HREF="https://arxiv.org/abs/1906.00899">arXiv:1906.00899</A> explains it as an abbreviation of *“not-necessarily-nilpotent”* (or $3n$-) displays. See also <A HREF="http://www.numdam.org/article/SB_2005-2006__48__341_0.pdf">Travaux de Zink</A>, page 343.