The Collaborative International Dictionary of English v.0.48:

Lambkill \Lamb"kill`\, n. (Bot.)
   A small American ericaceous shrub (Kalmia angustifolia),
   resembling mountain laurel but having narrower leaves and
   small red flowers; -- called also calfkill, sheepkill,
   sheep laurel, etc. It is supposed to poison young sheep and
   other animals that eat it at times when the snow is deep and
   they cannot find other food.

   Syn: sheep laurel, pig laurel, Kalmia angustifolia.
        [Webster 1913 Suppl. + WordNet 1.5]

WordNet (r) 3.0 (2006):

lambkill
    n 1: North American dwarf shrub resembling mountain laurel but
         having narrower leaves and small red flowers; poisonous to
         young stock [syn: sheep laurel, pig laurel, lambkill,
         Kalmia angustifolia]