The Collaborative International Dictionary of English v.0.48:

Thready \Thread"y\, a.
   1. Like thread or filaments; slender; as, the thready roots
      of a shrub.
      [1913 Webster]

   2. Containing, or consisting of, thread.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thready
    adj 1: forming viscous or glutinous threads [syn: ropy,
           ropey, stringy, thready]
    2: thin in diameter; resembling a thread [syn: filamentous,
       filiform, filamentlike, threadlike, thready]