Source : Webster's Revised Unabridged Dictionary (1913)

Dogwatch \Dog"watch`\, n. (Naut.)
   A half watch; a watch of two hours, of which there are two,
   the first dogwatch from 4 to 6 o'clock, p. m., and the second
   dogwatch from 6 to 8 o'clock, p. m. --Totten.

Source : WordNet®

dogwatch
     n : either of two short watches: from 4-6 pm or 6-8 pm