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Stephen, unfortunately Meigen did not give etymology. And a few species were described originally in this genus with feminine ending (none with masculine one):
Sybistroma americana Schiner, 1868,
Sybistroma setosa Schiner, 1862,
Sybistroma lenkoranica Negrobov, 1979
(http://members.fortunecity.com/grichanov/wScorpiurus.html#Sybistroma)
Henk Meuffels (teacher of Latin) has not found Greek/Latin meenings for this word
(http://members.fortunecity.com/grichanov/aas.htm)