FNU LNU (No Relation to GNU)

I just ran across these terms in United States v. FNU LNU (2d Cir., decided today), though a search reveals that one or both have been used in Westlaw-searchable case captions over 950 times since 1992 (but only 12 times before). All but 13 references have been in federal cases. (For a related item, see the PPA post.)