Etymology
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diphthong (n.)

"a union of two vowels pronounced in one syllable," late 15c., diptonge, from Late Latin diphthongus, from Greek diphthongos "having two sounds," from di- "double" (from PIE root *dwo- "two") + phthongos "sound, voice," which is related to phthengesthai "to utter a sound, sound, raise one's voice, call, talk," which Beekes reports as of "no certain etymology. None of the existing connections with semantically comparable words ... is phonetically convincing." Related: Diphthongal; diphthongization.

In uttering a proper diphthong both vowels are pronounced; the sound is not simple, but the two sounds are so blended as to be considered as forming one syllable, as in joy, noise, bound, out. An "improper" diphthong is not a diphthong at all, being merely a collocation of two or more vowels in the same syllable, of which only one is sounded, as ea in breach, eo in people, ai in rain, eau in beau. [Century Dictionary]
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dipsomania (n.)

1843, "morbid craving for alcohol," also used of the temporary madness caused by excessive drinking, coined in medical Latin from Greek dipsa "thirst" (which is of unknown origin) + mania. It is recorded earlier in Italian (1829) and German (1830) medical works.

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diphtheria (n.)

infectious disease, formerly frequently fatal, 1857, from French diphthérie, coined 1855 by physician Pierre Bretonneau (1778-1862) from Greek diphthera "prepared hide, leather," which is of unknown origin; the disease so called for the tough membrane that forms in the throat.

Bretonneau's earlier name for it was diphthérite (1821), which had been Englished as diphtheritis (1826). Related: Diphtheritic.

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diploma (n.)

1640s, "state paper, official document," from Latin diploma (plural diplomata) "a state letter of recommendation," given to persons travelling to the provinces, "a document drawn up by a magistrate," from Greek diploma "licence, chart," originally "paper folded double," from diploun "to double, fold over," from diploos "double" (see diplo-) + -oma, suffix forming neuter nouns and nouns that indicate result of verbal action (see -oma).

The main modern use is a specialized one, "a writing under seal from competent authority conferring some honor or privilege," especially that given by a college conferring a degree or authorizing the practice of a profession (1680s in English).

The plural is always -mas in the ordinary sense (certificate of degree &c.), though -mata lingers in unusual senses (state paper &c.) as an alternative. [Fowler]

Compare diplomacy, diplomatic.

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diplomacy (n.)

"the science of formal intercourse between nations through authorized agents; the art of negotiating and drafting treaties;" more loosely, "transactions and management of international business in general," 1793, from French diplomatie, formed from diplomate "diplomat" (on model of aristocratie from aristocrate), from Modern Latin diplomaticus (1680s), from Latin diploma (genitive diplomatis) "official document conferring a privilege" (see diploma; for sense evolution, see diplomatic).

It is obvious to any one who has been in charge of the interests of his country abroad that the day secrecy is abolished negotiations of any kind will become impossible. [Jules Cambon, "The Diplomatist" (transl. Christopher Rede Turner), 1931]

Meaning "dexterity or skill in managing negotiations of any kind" is by 1848.

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diptych (n.)

1620s, "hinged, two-leaved tablet of wood, ivory, etc., with waxed inner surfaces, used by the Greeks and Romans for writing with the style," from Latin diptycha (plural), from late Greek diptykha, neuter plural of diptykhos "double-folded, doubled," from di- "two" (see di- (1)) + ptykhe "fold," which is of uncertain etymology. In art, "a pair of pictures or carvings on two panels hinged together," by 1852.

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dipsomaniac (n.)

"drunkard, one suffering from an uncontrollable craving for strong drink," 1844, from dipsomania on the model of maniac. Slang shortening dipso is attested by 1880.

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diplo- 

before vowels dipl-, word-forming element of Greek origin, from Greek diploos, diplous "twofold, double," from di- "two" (see di- (1)) + -ploos "-fold," from PIE root *pel- (2) "to fold."

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diplodocus (n.)

genus of long-necked, long-tailed Jurassic dinosaurs, 1884, coined in Modern Latin in 1878 by U.S. paleontologist Othniel Charles Marsh (1831-1899) from Greek diploos "double" (see diplo-) + dokos "a bearing-beam," connected to dekomai "to take, accept, receive," as an agent-noun, so, properly, "which takes on (the load or covering)," from suffixed form of PIE root *dek- "to take, accept." The dinosaur was so called for the peculiar structure of the tail bones.

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diploid (adj.)

in cellular biology, "having two homologous sets of chromosomes," 1908, from German (1905), from Greek diploos "double, twofold," (see diplo-) + eidos "form" (see -oid).

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