The Collaborative International Dictionary of English v.0.48:

Tocsin \Toc"sin\, n. [F., fr. OF. toquier to touch, F. toquer
   (originally, a dialectic form of F. toucher) + seint (for
   sein) a bell, LL. signum, fr. L. signum a sign, signal. See
   Touch, and Sign.]
   An alarm bell, or the ringing of a bell for the purpose of
   alarm.
   [1913 Webster]

         The loud tocsin tolled their last alarm. --Campbell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tocsin
    n 1: the sound of an alarm (usually a bell) [syn: tocsin,
         alarm bell]
    2: a bell used to sound an alarm [syn: tocsin, warning bell]