Etymology
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in absentia (adv.)
Latin, literally "in (his/her/their) absence" (see absence). By 1831 in English, earlier in legal Latin.
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in vitro 
1892, scientific Latin; "in a test tube, culture dish, etc.;" literally "in glass," from Latin vitrum "glass" (see vitreous).
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stick-in-the-mud (n.)
1852, from verbal phrase, stick (v.) on notion of "one who sticks in the mud," hence "one who is content to remain in an abject condition." The phrase appears in 1730, in city of London court records, as the alias of an accused named John Baker, who with two other men received a death sentence at the Old Bailey in December 1733 for "breaking open the House of Mr. Thomas Rayner, a Silversmith, and stealing thence Plate to a great Value."
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tongue-in-cheek (adv.)

1856, from phrase to speak with one's tongue in one's cheek "to speak insincerely" (1748), suggestive of sly irony or humorous insincerity, perhaps a stage trick to convey irony to the audience.

Hem! Pray, Sir, said he to the Bard, after thrusting his Tongue into a Corner of his Cheek, and rolling his Eyes at Miss Willis, (Tricks which he had caught by endeavouring to take off a celebrated Comedian) were these fine Tragedies of yours ever acted? [anonymous, "Emily, or the History of a Natural Daughter," 1761]
This arietta, however, she no sooner began to perform, than he and the justice fell asleep ; but the moment she ceased playing, the knight waked snorting, and exclaimed,—'O cara! what d'ye think, gentlemen? Will you talk any more of your Pargolesi and your Corelli ?'—At the same time, he thrust his tongue in one cheek, and leered with one eye at the doctor and me, who sat on his left hand—He concluded the pantomime with a loud laugh, which he could command at all times extempore. [Smollett, "The Expedition of Humphrey Clinker," 1771]
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in forma pauperis 
legal Latin, literally "in the form of a poor person" (thus exempt from certain court fees, etc.), 1590s; see form (n.) + pauper (n.).
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in custodia legis (adv.)
legal Latin, "in the custody of the law," from ablative of custodia "a guarding, watching, keeping" (see custody) + legis, genitive of lex "law" (see legal (adj.)).
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jack-in-the-box (n.)
also jack-in-a-box, 1560s, a name for a sharp or cheat, "who deceived tradesmen by substituting empty boxes for others full of money" [Robert Nares, "A Glossary of Words, Phrases, Names, and Allusions," London, 1905]. See Jack + box (n.1). As a type of toy involving a figure on a spring inside a box, it is attested by 1702. Also it has been used variously to mean "peddler who sells wares from a temporary stall" (1690s), "an unborn child," a type of gambling game, a hermit crab, a large wooden male screw, the sacrament, and various mechanical devices.
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brother-in-law (n.)
"brother of one's husband or wife," also "brother of one's sister's husband," c. 1300; also brother in law; see brother + in-law. In Arabic, Urdu, Swahili, etc., brother-in-law, when addressed to a male who is not a brother-in-law, is an extreme insult, with implications of "I slept with your sister."
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