The Collaborative International Dictionary of English v.0.48:

Teeth \Teeth\, v. i. [imp. & p. p. Teethed; p. pr. & vb. n.
   Teething.]
   To breed, or grow, teeth.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Teething \Teeth"ing\, n.
   The process of the first growth of teeth, or the phenomena
   attending their issue through the gums; dentition.
   [1913 Webster]

WordNet (r) 3.0 (2006):

teething
    n 1: the eruption through the gums of baby teeth [syn:
         teething, dentition, odontiasis]