The Collaborative International Dictionary of English v.0.48:

Cudweed \Cud"weed`\ (k?d"w?d`), n. [Apparently fr. cud + weed,
   but perh. a corruption of cottonweed; or of cut weed, so
   called from its use as an application to cuts and chafings.]
   (Bot.)
   A small composite plant with cottony or silky stem and
   leaves, primarily a species of Gnaphalium, but the name is
   now given to many plants of different genera, as Filago,
   Antennaria, etc.; cottonweed.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cudweed
    n 1: any of numerous plants of the genus Gnaphalium having
         flowers that can be dried without loss of form or color
    2: any plant of the genus Filago having capitate clusters of
       small woolly flower heads [syn: cotton rose, cudweed,
       filago]
    3: perennial cottony-white herb of southwestern United States
       [syn: western mugwort, white sage, cudweed, prairie
       sage, Artemisia ludoviciana, Artemisia gnaphalodes]