Source : Webster's Revised Unabridged Dictionary (1913)

Tread-softly \Tread"-soft`ly\, n. (Bot.)
   Spurge nettle. See under {Nettle}.

Source : WordNet®

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