"tubular instrument inserted to draw off urine from the bladder," c. 1600, from French cathéter, from Late Latin catheter "a catheter," from Greek katheter "surgical catheter," literally "anything let down," from stem of kathienai "to let down, thrust in," from kata "down" (see cata-) + stem of hienai "to send" (from PIE root *ye- "to throw, impel").
Earlier was cathirum (early 15c.), directly from Medieval Latin; in this sense Middle English also had argalia, via Medieval Latin from Arabic. Related: Catheterization; catheterized; catheterizing.
1630s, "authoritative sanction," from Latin fiat "let it be done" (used in the opening of Medieval Latin proclamations and commands), third person singular present subjunctive of fieri "be done, become, come into existence" (from PIE root *bheue- "to be, exist, grow"), used as passive of facere "to make, do." Meaning "a decree, command, order" is from 1750. In English the word also sometimes is a reference to fiat lux "let there be light" in Genesis i.3.
Dixitque Deus: Fiat lux. Et facta est lux. [Vulgate]
"place or opening by which anything is let out or escapes," mid-13c., "a river mouth," from out- + let (v.). Electrical wiring sense, "socket that connects a device to an electricity supply," is attested from 1892. The commercial sense of "a market for the sale of any product" is by 1889; that of "a retail store disposing of a manufacturer's products" is attested by 1933. Figurative sense "means of relief or discharge" is from 1620s.
mid-14c., omissioun, "a neglect or failure to do what one has power to do or ought to do," from Anglo-French omission (early 14c.), Old French omission and directly from Late Latin omissionem (nominative omissio) "an omitting," noun of action from past-participle stem of Latin omittere "disregard," literally "let go, let fall" (see omit). Meaning "act of leaving out" is from 1550s. Related: Omissible.
proverbial for "something extremely rare or non-existent" (late 14c.) is from Juvenal ["Sat." vi. 164], but the real thing turned up in Australia (Chenopsis atratus).
"Do you say no worthy wife is to be found among all these crowds?" Well, let her be handsome, charming, rich and fertile; let her have ancient ancestors ranged about her halls; let her be more chaste than all the dishevelled Sabine maidens who stopped the war—a prodigy as rare upon the earth as a black swan! yet who could endure a wife that possessed all perfections? I would rather have a Venusian wench for my wife than you, O Cornelia, mother of the Gracchi, if, with all your virtues, you bring me a haughty brow, and reckon up Triumphs as part of your marriage portion. [Juvenal]
Blue dahlia also was used 19c. for "something rare and unheard of."