Source : Webster's Revised Unabridged Dictionary (1913)

Goaf \Goaf\; n.; pl. {Goafs}or {Goaves}. [Cf. lst {Gob}.]
   (Mining)
   That part of a mine from which the mineral has been partially
   or wholly removed; the waste left in old workings; -- called
   also {gob} .

   {To work the goaf} or {gob}, to remove the pillars of mineral
      matter previously left to support the roof, and replace
      them with props. --Ure.