• (n.) A lantern, or louver covering, placed on a roof, for ventilation or escape of smoke.
Fem'er·ell noun [ Old French fumeraille part of a chimney. See Fume .] (Architecture) A lantern, or louver covering, placed on a roof, for ventilation or escape of smoke. 
In architecture a femerell is a lantern, or louvre covering, placed on a roof, for ventilation or the escape of smoke.