The Collaborative International Dictionary of English v.0.48:

Onagraceous \On`a*gra"ceous\ ([o^]n`[.a]*gr[=a]"sh[u^]s),
Onagrarieous \On`a*gra*ri"e*ous\ (-gr[asl]*r[imac]"[-e]*[u^]s),
   a. [From NL. Onagra an old scientific name of the evening
   primrose ([OE]nothera), fr. Gr. 'ona`gra a kind of plant;
   of uncertain origin.] (Bot.)
   Pertaining to, or resembling, a natural order of plants
   (Onagraceae or Onagrarieae), which includes the fuchsia,
   the willow-herb (Epilobium), and the evening primrose
   ([OE]nothera).
   [1913 Webster]