Word Study
entresol
CIDE DICTIONARY
entresol, n. [F.].
A low story between two higher ones, usually between the ground floor and the first story; mezzanine. Parker. [1913 Webster]
OXFORD DICTIONARY
entresol, n. a low storey between the first and the ground floor; a mezzanine floor.
Etymology
F f. entre between + sol ground
For further exploring for "entresol" in Webster Dictionary Online