From The Collaborative International Dictionary of English v.0.48 [gcide]:
Midriff \Mid"riff\ (m[i^]d"r[i^]f), n. [AS. midhrif; midd mid,
middle + hrif bowels, womb; akin to OFries. midref midriff,
rif, ref, belly, OHG. href body, and to L. corpus body. See
Corpse.] (Anat.)
1. See Diaphragm, n., 2.
[1913 Webster]
2. The middle part of the front of the body, from the waist
to the chest.
[PJC]
Smote him into the midriff with a stone. --Milton.
[1913 Webster]
3. That part of a garment, especially a dress or bodice, that
covers the midriff[2].
[PJC] Mid sea
From WordNet (r) 3.0 (2006) [wn]:
midriff
n 1: the middle area of the human torso (usually in front);
"young American women believe that a bare midriff is
fashionable" [syn: middle, midriff, midsection]
2: (anatomy) a muscular partition separating the abdominal and
thoracic cavities; functions in respiration [syn:
diaphragm, midriff]