Etymology
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nerve (v.)
c. 1500, "to ornament with threads;" see nerve (n.). Meaning "to give strength or vigor" is from 1749. Related: Nerved; nerving.
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sixth (adj., n.)

"next in order after the fifth; an ordinal numeral; being one of six equal parts into which a whole is regarded as divided;" 1520s, replacing Middle English sixte (c. 1200), from Old English syxte, from siex (see six). Compare Old Frisian sexta, Middle Dutch seste, Old High German sehsto, German sechste, Gothic saihsta. With ending conformed to -th (1). Related: Sixthly. The noun meaning "a sixth part" is from 1550s. As a music tone, from 1590s. Sixth sense "supernatural perception of objects" is attested from 1712; earlier it meant "titillation, the sense that apprehends sexual pleasure" (1690s, from Scaliger).

Then said Peter, That is false; for there is a sixth Sense, that of Prescience : for the other five Senses are capable only of Knowledg ; but the Sixth of Foreknowledg ; which Sense the Prophets had. [William Whitson, "Primitive Christianity Reviv'd," vol. v, London, 1712]
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cranial (adj.)

"pertaining to the skull," 1779, from Modern Latin cranium, from Greek kranion "skull" (see cranium) + -al (1).

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nerve (n.)

late 14c., nerve, nerf, "sinew, tendon, hard cord of the body" (a sense now obsolete), also "fiber or bundle of fibers that convey the capacity to feel or move from the brain or spinal cord to the body," from Old French nerf and directly from Medieval Latin nervus "a nerve," from Latin nervus "sinew, tendon; cord, bowstring, string of a musical instrument," metathesis of pre-Latin *neuros, from PIE *(s)neu- "tendon, sinew" (source also of Sanskrit snavan- "band, sinew," Armenian neard "sinew," Greek neuron "sinew, tendon," in Galen "nerve").

The late medieval surgeons understood the nature and function of the nerves and often used nervus to denote a `nerve' in the modern sense, as well as to denote a `tendon'. There appears to have been some confusion, however, between nerves and tendons; hence, a number of instances in which nervus may be interpreted in either way or in both ways simultaneously. [Middle English Compendium] 

The secondary senses developed from meaning "strength, vigor; force, energy" (c. 1600), from the "sinew" sense. Hence the non-scientific sense with reference to feeling or courage, first attested c. 1600 (as in nerves of steel, 1869) and that of "coolness in the face of danger, fortitude under trying or critical circumstances" is by 1809. The bad sense "impudence, boldness, cheek" (originally slang) is by 1887. Latin nervus also had a figurative sense of "vigor, force, power, strength," as did Greek neuron. From the neurological sense come Nerves "condition of hysterical nervousness," attested by 1890, perhaps from 1792. To get on (someone's) nerves is from 1895. War of nerves "psychological warfare" is from 1915.

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nerve-wracking (adj.)

also nervewracking, 1867, from nerve (n.) + present participle of wrack (v.). See nerve-racking.

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nerve-racking (adj.)

also nerveracking, "causing anxiety or mental stress," 1812, from nerve (n.) + present participle of rack (v.1). Between nerve-racking and nerve-wracking (1867) this is probably the better choice as a figure of speech, but the sense of wrack (v.), though less suitable in the image, is not obviously wrong.

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Sextus 

masc. proper name, from Latin, properly "the sixth," originally denoting a sixth child, from sextus "sixth," from sex "six" (see six; compare Octavian).

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zeta (n.)
sixth letter of the Greek alphabet; see zed.
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neuritis (n.)
"inflammation of a nerve or nerves," 1825, from Greek neuron "nerve" (see neuro-) + -itis "inflammation." Related: Neuritic.
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neural (adj.)

"pertaining to a nerve or nerves, pertaining to the nervous system generally," 1830, from Greek neuron "nerve" (see neuro-) + adjectival suffix -al (1). Related: Neurally. Compare nervous.

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