NOUN (3)

1. the time that the curfew signal is sounded;

2. a signal (usually a bell) announcing the start of curfew restrictions;

3. an order that after a specific time certain activities (as being outside on the streets) are prohibited;

The Collaborative International Dictionary of English v.0.48:

Curfew \Cur"few\ (k[^u]r"f[=u]), n. [OE. courfew, curfu, fr. OF.
   cuevrefu, covrefeu, F. couvre-feu; covrir to cover + feu
   fire, fr. L. focus fireplace, hearth. See Cover, and
   Focus.]
   1. The ringing of an evening bell, originally a signal to the
      inhabitants to cover fires, extinguish lights, and retire
      to rest, -- instituted by William the Conqueror; also, the
      bell itself.
      [1913 Webster]

            He begins at curfew, and walks till the first cock.
                                                  --Shak.
      [1913 Webster]

            The village curfew, as it tolled profound.
                                                  --Campbell.
      [1913 Webster]

   2. A utensil for covering the fire. [Obs.]
      [1913 Webster]

            For pans, pots, curfews, counters and the like.
                                                  --Bacon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

curfew
    n 1: the time that the curfew signal is sounded
    2: a signal (usually a bell) announcing the start of curfew
       restrictions
    3: an order that after a specific time certain activities (as
       being outside on the streets) are prohibited

Bouvier's Law Dictionary, Revised 6th Ed (1856):

CURFEW.  The name of a law, established during the reign of the English
king, William, the conqueror, by which the people were commanded to dispense
with fire and candle at eight o'clock at night.
     It was abolished in the reign of Henry I., but afterwards it signified
the time at which the curfew formerly took place. The word curfew is
derived, probably, from couvre few, or cover fire. 4 Bl. Com. 419, 420.