Old English midhrif "diaphragm of a human or animal," from mid "mid" (from PIE root *medhyo- "middle") + hrif "belly," from Proto-West Germanic *hrefin (source also of Old High German href, Old Frisian hrif "belly"), from PIE root *kwrep- "body, form, appearance."
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.
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