Source : Webster's Revised Unabridged Dictionary (1913)

Gruel \Gru"el\, n. [OF. gruel, F. gruau; of German origin; cf.
   OHG. gruzzi groats, G. gr["u]tze, As. grut. See {Grout}.]
   A light, liquid food, made by boiling meal of maize, oatmeal,
   or fiour in water or milk; thin porridge.

Source : WordNet®

gruel
     n : a thin porridge (usually oatmeal or cornmeal)