The Collaborative International Dictionary of English v.0.48:

Groundsel \Ground"sel\, n. [OE. grundswilie, AS. grundeswylige,
   grundeswelge, earlier gundiswilge; gund matter, pus + swelgan
   to swallow. So named as being good for a running from the
   eye. See Swallow, v.] (Bot.)
   An annual composite plant (Senecio vulgaris), one of the
   most common and widely distributed weeds on the globe.
   Groundsel

The Collaborative International Dictionary of English v.0.48:

Groundsel \Ground"sel\, Groundsill \Ground"sill`\, n. [Ground +
   sill.]
   See Ground plate
   (a), under Ground
       [1913 Webster]

WordNet (r) 3.0 (2006):

groundsel
    n 1: Eurasian weed with heads of small yellow flowers [syn:
         groundsel, Senecio vulgaris]