Etymology
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deepness (n.)

Old English deopnes "deep water," also "a mystery or secret;" see deep (adj.) + -ness. From late 12c. as "distance downward;" c. 1200 as "wisdom, profundity."

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biblioklept (n.)
"one who steals books," 1880, from biblio- "book" + Greek kleptes "thief" (see kleptomania). Walsh calls it "a modern euphemism which softens the ugly word book-thief by shrouding it in the mystery of the Greek language."
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apiary (n.)

1650s, from Latin apiarium "bee-house, beehive," noun use of neuter of apiarius "of bees," from apis "bee," a mystery word unrelated to any similar words in other Indo-European languages. A borrowing from Semitic has been proposed. Related: Apiarian (1798).

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denouement (n.)

"the solution of a mystery, the winding up of a plot, the outcome of a course of conduct," 1752, from French dénouement "an untying" (of plot), from dénouer "untie" (Old French desnouer) from des- "un-, out" (see dis-) + nouer "to tie, knot," from Latin nodus "a knot," from PIE root *ned- "to bind, tie."

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profundity (n.)

early 15c., "bottom of the sea," from Old French profundite (Modern French profondité) and directly from Late Latin profunditatem (nominative profunditas) "depth, intensity, immensity," from profundus "deep, vast" (see profound). Meaning "depth of intellect, feeling, or spiritual mystery" in English is from c. 1500.

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arcana (n.)
"hidden things, mysteries," 1590s, a direct adoption of the Latin plural of arcanum "a secret, a mystery," an important word in alchemy, from neuter of adjective arcanus "secret, hidden, private, concealed" (see arcane). Occasionally mistaken for a singular and pluralized as arcanas, because arcana is far more common than arcanum.
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privacy (n.)

1590s, "a private or personal matter, a secret;" c. 1600 as "seclusion, state of being in retirement from company or the knowledge and observation of others," from private (adj.) + abstract noun suffix -cy. Meaning "state of freedom from intrusion or interference" is from 1814. Earlier was privatie (late 14c. as "secret, mystery;" c. 1400 as "a secret, secret deed; solitude, privacy"), from Old French privauté.

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interlude (n.)

formerly also enterlude, c. 1300, from Old French entrelude and directly from Medieval Latin interludium "an interlude," from Latin inter "between" (see inter-) + ludus "a play" (see ludicrous). Originally the term for farcical episodes ("generally short and coarse" - Century Dictionary) drawn from real life introduced between acts of long mystery or morality plays. In 17c.-18c. it meant "popular stage play;" transferred (non-dramatic) sense of "interval in the course of some action" is from 1751. Related: Interludial.

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hiker (n.)

1908, agent noun from hike (v.). Earlier as a type of boat:

The "hiker" or "tuck-up" as it is more generally termed, is a craft peculiar to the Delaware River, and is to the youth residing along the banks of that stream what the racing shell is to the Torontonian .... The origin of the name "hiker" is veiled in mystery. No member of the clubs engaged in sailing these boats can give anything like a satisfactory derivation of the word. The most common explanation is that it is corrupted from the local verb "to hike," which means to run or fly swiftly. ["Harper's Young People," 1885]
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lieutenant (n.)
late 14c., "one who takes the place of another," from Old French lieu tenant "substitute, deputy," literally "place holder" (14c.), from lieu "place" (see lieu) + tenant, present participle of tenir "to hold," from PIE root *ten- "to stretch." The notion is of a "substitute" for higher authority.

Specific military sense of "army officer next in rank to a captain and commanding the company in his absence" is from 1570s. Pronunciation with lef- is common in Britain, and spellings to reflect it date back to 14c., but the origin of this is a mystery (OED rejects suggestion that it comes from old confusion of -u- and -v-).
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