The Collaborative International Dictionary of English v.0.48:

Cuckooflower \Cuck"oo*flow`er\ (-flou`?r), n. (Bot.)
   A species of Cardamine (Cardamine pratensis), or lady's
   smock. Its leaves are used in salads. Also, the ragged robin
   (Lychnis Flos-cuculi).
   [1913 Webster]

WordNet (r) 3.0 (2006):

cuckooflower
    n 1: a bitter cress of Europe and America [syn: lady's smock,
         cuckooflower, cuckoo flower, meadow cress, Cardamine
         pratensis]