The Collaborative International Dictionary of English v.0.48:

Tread-softly \Tread"-soft`ly\, n. (Bot.)
   Spurge nettle. See under Nettle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tread-softly
    n 1: a stinging herb of tropical America [syn: spurge nettle,
         tread-softly, devil nettle, pica-pica, Cnidoscolus
         urens, Jatropha urens, Jatropha stimulosus]