The Collaborative International Dictionary of English v.0.48:

darnel \dar"nel\ (d[a^]r"n[e^]l), n. [OE. darnel, dernel, of
   uncertain origin; cf. dial. F. darnelle, Sw. d[*a]r-repe;
   perh. named from a supposed intoxicating quality of the
   plant, and akin to Sw. d[*a]ra to infatuate, OD. door
   foolish, G. thor fool, and Ee. dizzy.] (Bot.)
   Any grass of the genus Lolium, esp. the Lolium temulentum
   (bearded darnel), the grains of which have been reputed
   poisonous. Other species, as Lolium perenne (rye grass or
   ray grass), and its variety Lolium Italicum (Italian rye
   grass), are highly esteemed for pasture and for making hay.
   [1913 Webster]

   Note: Under darnel our early herbalists comprehended all
         kinds of cornfield weeds. --Dr. Prior.
         [1913 Webster]

WordNet (r) 3.0 (2006):

darnel
    n 1: weedy annual grass often occurs in grainfields and other
         cultivated land; seeds sometimes considered poisonous [syn:
         darnel, tare, bearded darnel, cheat, Lolium
         temulentum]