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Old English midhrif, from mid "mid" (see mid) + hrif "belly," from Proto-Germanic *hrefiz- (cf. Old High German href, Old Frisian hrif "belly"), from PIE *kwrep- "body, form, appearance" (see corporeal). More or less obsolete after 18c. except in phrase to tickle (one's) midriff "to cause laughter," the word revived 1941 in fashion usage for "portion of a woman's garment that covers the belly," as a euphemistic avoidance of belly; sense inverted and extended 1970 to a belly-baring style of women's top.
midriff mid·riff (mĭd'rĭf)