Source : Webster's Revised Unabridged Dictionary (1913)

Countinghouse \Count"ing*house`\ (kount"?ng-hous`), Countingroom
\Count"ing*room`\ (kount"?ng-r??m`), n. [See {Count}, v.]
   The house or room in which a merchant, trader, or
   manufacturer keeps his books and transacts business.

Source : WordNet®

countinghouse
     n : office used by the accountants of a business