The Collaborative International Dictionary of English v.0.48:

Lion's tail \Li"on's tail`\ (l[imac]"[u^]nz t[=a]l`) n. (Bot.)
   A genus of labiate plants (Leonurus); -- so called from a
   fancied resemblance of its flower spikes to the tuft of a
   lion's tail. Leonurus Cardiaca is the common motherwort.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Motherwort \Moth"er*wort`\, n. (Bot.)
   (a) A labiate herb (Leonurus Cardiaca), of a bitter taste,
       used popularly in medicine; lion's tail.
   (b) The mugwort. See Mugwort.
       [1913 Webster]

WordNet (r) 3.0 (2006):

Leonurus cardiaca
    n 1: bitter Old World herb of hedgerows and woodland margins
         having toothed leaves and white or pale pink flowers [syn:
         motherwort, Leonurus cardiaca]