Author of genus is
Brauer, 1868. From nanno (or
nano, from Greek, meaning little old man, or dwarf), plus the ubiquitous suffix
-themis. This is from Greek, first used by
Hagen in 1861, and it means "what has been established", i.e., rule, or law.
Themis was also a goddess of order and justice. However, it is possible the suffix was also simply based on the Greek suffixes
-hemis (half), or
-emis. See this
Odonata list post by KD Dijkstra.