Etymology
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rectum (n.)

in anatomy, "the terminal section of the intestine, ending in the anus," early 15c., from Latin intestinum rectum "straight intestine" (in contrast to the convolution of the rest of the bowels), from neuter past participle of regere "to keep straight" (from PIE root *reg- "move in a straight line"). A loan-translation of Greek apeuthysmeon enteron, "the name given to the lowest part of the large intestine by Galen, who so called it because he dissected only animals whose rectum (in contradistinction to that of man) is really straight" [Klein].

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rectal (adj.)

"pertaining to or connected with the rectum," 1822, from stem of rectum + -al (1). Related: Rectally.

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recto- 

word-forming element in anatomy meaning "rectal, pertaining to or involving the rectum," before vowels rect-, from combining form of rectum.

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*reg- 

Proto-Indo-European root meaning "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule."

It forms all or part of: abrogate; address; adroit; Alaric; alert; anorectic; anorexia; arrogant; arrogate; bishopric; correct; corvee; derecho; derogate; derogatory; Dietrich; direct; dress; eldritch; erect; ergo; Eric; Frederick; Henry; incorrigible; interregnum; interrogate; maharajah; Maratha; prerogative; prorogue; rack (n.1) "frame with bars;" rail (n.1) "horizontal bar passing from one post or support to another;" Raj; rajah; rake (n.1) "toothed tool for drawing or scraping things together;" rake (n.2) "debauchee; idle, dissolute person;" rakish; rank (adj.) "corrupt, loathsome, foul;" real (n.) "small Spanish silver coin;" realm; reck; reckless; reckon; rectangle; rectify; rectilinear; rectitude; recto; recto-; rector; rectum; regal; regent; regicide; regime; regimen; regiment; region; regular; regulate; Regulus; Reich; reign; resurgent; rex; rich; right; Risorgimento; rogation; royal; rule; sord; source; subrogate; subrogation; surge; surrogate; viceroy.

It is the hypothetical source of/evidence for its existence is provided by:

Sanskrit raj- "a king, a leader," rjyati "he stretches himself," riag "torture" (by racking); Avestan razeyeiti "directs," raštva- "directed, arranged, straight;" Persian rahst "right, correct;" Latin regere "to rule, direct, lead, govern," rex (genitive regis) "king," rectus "right, correct;" Greek oregein "to reach, extend;" Old Irish ri, Gaelic righ "a king," Gaulish -rix "a king" (in personal names, such as Vircingetorix), Old Irish rigim "to stretch out;" Gothic reiks "a leader," raihts "straight, right;" Lithuanian raižytis "to stretch oneself;" Old English rice "kingdom," -ric "king," rice "rich, powerful," riht "correct;" Gothic raihts, Old High German recht, Old Swedish reht, Old Norse rettr "correct."

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colorectal (adj.)

"pertaining to the colon and the rectum," by 1918, from combining form of colon (n.2) + rectal.

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recto (n.)
"right-hand page in an open book" (opposed to verso or reverso), 1824, from Latin recto (in recto folio), ablative of rectum "right" (see right (adj.2)).
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proctalgia (n.)

"pain in the anus or rectum," 1811, from medical Latin proct-, Latinized form of Greek combining form of prōktos "anus" (see proctology) + algos "pain" (see -algia).

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proctology (n.)

"branch of medicine concerned with the anus or rectum," 1896, from Latinized form of Greek prōktos "anus" (from PIE *prokto-, source also of  Armenian erastan-k' "buttocks") + -logy "study of." Related: Proctologist (1897).

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