Etymology
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midriff (n.)

Old English midhrif "diaphragm of a human or animal," from mid "mid" (from PIE root *medhyo- "middle") + hrif "belly," from Proto-Germanic *hrefin (source also of Old High German href, Old Frisian hrif, -rith, -rede "belly"). Compare Old Frisian midrede "diaphragm." Watkins has this from PIE root *kwrep- "body, form, appearance;" Boutkan has it from *sker- (1) "to cut."

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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*kwrep- 
Proto-Indo-European root meaning "body, form, appearance," probably a verbal root meaning "to appear."

It forms all or part of: corporal (adj.) "of or belonging to the body;" corporate; corporation; corporeal; corps; corpse; corpulence; corpulent; corpus; corpuscle; corsage; corse; corset; incorporeal; incorporate; leprechaun; midriff.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit krp- "form, body;" Avestan kerefsh "form, body;" Latin corpus "body" (living or dead); Old English hrif "belly," Old High German href "womb, belly, abdomen."
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diaphragm (n.)

late 14c., diafragma, in anatomy, "muscular membrane which separates the thorax from the abdominal cavity in mammals," from Late Latin diaphragma, from Greek diaphragma "partition, barrier, muscle which divides the thorax from the abdomen," from diaphrassein "to barricade," from dia "across" (see dia-) + phrassein "to fence or hedge in," which is of uncertain etymology. Beekes suggests it is a substrate word and finds "no convincing correspondence outside Greek."

The native word in the anatomical sense is midriff. From 1650s as "a partition" of any kind, "something which divides or separates;" 1660s in the special sense "thin piece of metal" serving some purpose (as a sound resonator, etc.). Meaning "contraceptive cap" is from 1933. Related: Diaphragmal; diaphragmatic.

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*medhyo- 
Proto-Indo-European root meaning "middle." Perhaps related to PIE root *me- (2) "to measure."

It forms all or part of: amid; intermediate; mean (adj.2) "occupying a middle or intermediate place;" medal; medial; median; mediate; medieval; mediocre; Mediterranean; medium; meridian; mesic; mesial; meso-; meson; Mesopotamia; Mesozoic; mezzanine; mezzo; mezzotint; mid (prep., adj.); middle; Midgard; midriff; midst; midwife; milieu; minge; mizzen; moiety; mullion.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit madhyah, Avestan madiya- "middle," Greek mesos, Latin medius "in the middle, between; from the middle," Gothic midjis, Old English midd "middle," Old Church Slavonic medzu "between," Armenian mej "middle."
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belly (n.)

a general Germanic word for "leather bag, pouch, pod" that in English has evolved to mean a part of the body; from Old English belg, bylig (West Saxon), bælg (Anglian) "leather bag, purse, pouch, pod, husk, bellows," from Proto-Germanic *balgiz "bag" (source also of Old Norse belgr "bag, bellows," bylgja "billow," Gothic balgs "wine-skin"), from PIE *bhelgh- "to swell," extended form of root *bhel- (2) "to blow, swell."

By c. 1200 it was being used for "the stomach," especially as a symbol of gluttony, and by late 14c. to mean "abdomen of a human or animal, front part of the body between the breast and the groin or the diaphragm and the pelvis."

The Old English word for "belly, stomach" was buc (cognate with German Bauch, Dutch buik, Old Frisian buk, from West Germanic *būkaz, a word indicative of swelling, with no known connections). The plural of Old English belg emerged in Middle English as a separate word, bellows. Meaning "bulging part or convex surface of anything" is 1590s. The West Germanic root had a figurative or extended sense of "anger, arrogance" (as in Old English bolgenmod "enraged;" belgan (v.) "to become angry"), probably from the notion of "swelling."

Indo-European languages commonly use the same word for both the external belly and the internal (stomach, womb, etc.), but the distinction of external and internal is somewhat present in English belly/stomach; Greek gastr- (see gastric) in classical language denoted the paunch or belly, while modern science uses it only in reference to the stomach as an organ.

As a personal name from 12c. Belly-naked in Middle English was "stripped to the belly, completely naked." Fastidious avoidance of belly in speech and writing (compensated for by stretching the senses of imported stomach and abdomen, baby-talk tummy and misappropriated midriff) began late 18c. and the word was banished from Bibles in many early 19c. editions.

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midsection (n.)

also mid-section, "middle of the human body, midriff, belly," by 1939, an advertiser's word, from mid (adj.) + section (n.).

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renal (adj.)

"of or pertaining to the kidneys," 1650s, from French rénal and directly from Late Latin renalis "of or belonging to kidneys," from Latin ren (plural renes) "kidneys," a word of of uncertain etymology, with possible cognates in Old Irish aru "kidney, gland," Welsh arenn "kidney, testicle," Hittite hah(ha)ari "lung(s), midriff." Also possibly related are Old Prussian straunay, Lithuanian strėnos "loins," Latvian streina "loins." "The semantic shift from 'loins' to 'kidneys' is quite conceivable" [de Vaan].

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phrase (n.)

1520s, "manner or style of expression," also "brief expression with some unity; two or more words expressing what is practically a single notion," from Late Latin phrasis "diction," from Greek phrasis "speech, way of speaking, enunciation, phraseology," from phrazein "to tell, declare, indicate, point out, show, inform," also passively (phrazomai), "indicate to oneself, think or muse upon, consider; think up, contrive; suppose, believe, imagine; perceive, observe."

The Greek verb is of uncertain origin; perhaps it is connected with phrenes "wits, senses, sanity," phrēn "the mind, the heart," literally "midriff, diaphragm" (see phreno-). The musical sense of "a short and somewhat independent passage from a piece" is from 1789. Phrase-book "collection of expressions peculiar to a language" is by 1590s.

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phreno- 

before vowels phren-, word-forming element meaning "mind," also, in medical use, "diaphragm, muscle which parts the abdomen from the thorax;" from Greek phrēn, phrenos "the mind, spirit," also "the midriff, diaphragm," also (in plural, phrenes) "the parts around the heart, the breast," and hence "wits, sense, sanity, mind" on the notion of the breast or heart as the seat of thoughts and passions.

The word is of uncertain origin; Watkins has it under a proposed PIE root ‌‌*gwhren- "to think." Beekes finds the connection with phrassein "to fence or hedge in" "semantically attractive," but there are phonetic difficulties, and he finds "quite feasible" a relationship with phrazomai "to think, consider" (later phrazein; see phrase (n.)), itself an isolated word.

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stomach (n.)

late 14c. variant of earlier stomake (early 14c.), "internal pouch into which food is digested," from Old French stomaque, estomac "stomach," from Latin stomachus "throat, gullet; stomach," also "taste, inclination, liking; distaste, dislike;" also "pride, indignation," which were thought to have their origin in that organ (source also of Spanish estómago, Italian stomaco), from Greek stomachos "throat, gullet, esophagus," literally "mouth, opening," from stoma "mouth" (see stoma).

Applied anciently to the openings of various internal organs, especially that of the stomach, then by the later Greek physicians to the stomach itself. The native word is maw (Old English maga glosses stomachus).

Some 16c. anatomists tried to correct the sense back to "esophagus" and introduce ventricle for what we call the stomach. Meaning "belly, midriff, part of the body that contains the stomach" is from late 14c.

In Middle English also stomack, stomac, stommak, stomoke; the spelling of the ending of the word was conformed to Latin regularly from 16c., but the pronunciation remains as in Middle English. Related: stomachial (1580s); stomachical (c. 1600); stomachic (1650s). Pugilistic stomacher "punch in the stomach" is from 1814; from mid-15c. as "vest or other garment which covers the belly." The Latin figurative senses also were in Middle English (such as "relish, inclination, desire," mid-15c.) or early Modern English. Also sometimes regarded in Middle Ages as the seat of sexual desire.

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