Etymology
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ineffable (adj.)
late 14c., "beyond expression, too great for words, inexpressible," from Old French ineffable (14c.) or directly from Latin ineffabilis "unutterable," from in- "not, opposite of" (see in- (1)) + effabilis "speakable," from effari "utter," from assimilated form of ex "out" (see ex-) + fari "to say, speak," from PIE root *bha- (2) "to speak, tell, say."

Meaning "that may not be spoken" is from 1590s. Plural noun ineffables was, for a time, a jocular euphemism for "trousers" (1823; see inexpressible). Related: Ineffably.
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effable (adj.)
"that may be (lawfully) expressed in words," 1630s, from French effable or directly, from Latin effabilis "utterable," from effari "to utter" (see ineffable). Now obsolete or archaic.
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*bha- (2)

*bhā-; Proto-Indo-European root meaning "to speak, tell, say."

It forms all or part of: abandon; affable; anthem; antiphon; aphasia; aphonia; aphonic; apophasis; apophatic; ban (n.1) "proclamation or edict;" ban (v.); banal; bandit; banish; banlieue; banns (n.); bifarious; blame; blaspheme; blasphemy; boon (n.); cacophony; confess; contraband; defame; dysphemism; euphemism; euphony; fable; fabulous; fado; fairy; fame; famous; fandango; fatal; fate; fateful; fatuous; fay; gramophone; heterophemy; homophone; ineffable; infamous; infamy; infant; infantile; infantry; mauvais; megaphone; microphone; monophonic; nefandous; nefarious; phatic; -phone; phone (n.2) "elementary sound of a spoken language;" phoneme; phonetic; phonic; phonics; phono-; pheme; -phemia; Polyphemus; polyphony; preface; profess; profession; professional; professor; prophecy; prophet; prophetic; quadraphonic; symphony; telephone; xylophone.

It is the hypothetical source of/evidence for its existence is provided by: Greek pheme "speech, voice, utterance, a speaking, talk," phōnē "voice, sound" of a human or animal, also "tone, voice, pronunciation, speech," phanai "to speak;" Sanskrit bhanati "speaks;" Latin fari "to say," fabula "narrative, account, tale, story," fama "talk, rumor, report; reputation, public opinion; renown, reputation;" Armenian ban, bay "word, term;" Old Church Slavonic bajati "to talk, tell;" Old English boian "to boast," ben "prayer, request;" Old Irish bann "law."

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Adonai 
Old Testament word for "God," used as a substitute for the ineffable name, late 14c., from Medieval Latin, from Hebrew, literally "my lord," from adon (see Adonis) + suffix of the first person.
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Jehovah 
1530, Tyndale's transliteration of Hebrew Tetragrammaton YHWH using vowel points of Adhonai "my lord" (see Yahweh). Used for YHWH (the full name being too sacred for utterance) in four places in the Old Testament in the KJV where the usual translation the lord would have been inconvenient; taken as the principal and personal name of God.

The vowel substitution was originally made by the Masoretes as a direction to substitute Adhonai for "the ineffable name." European students of Hebrew took this literally, which yielded Latin JeHoVa (first attested in writings of Galatinus, confessor to Leo X, 1516). Jehovah's Witnesses "member of Watchtower Bible and Tract Society" first attested 1933; the organization founded c. 1879 by Charles Taze Russell (1852-1916); the name from Isaiah xliii.10.
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