Etymology
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caution (v.)

"to warn, exhort to take heed," 1640s, from caution (n.). Related: Cautioned; cautioning.

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caution (n.)

c. 1300, caucioun, "bail, guarantee, pledge," from Old French caution "security, surety" (13c.), from Latin cautionem (nominative cautio) "caution, care, foresight, precaution," noun of action from past participle stem of cavere "to be on one's guard" (from PIE root *keu- "to see, observe, perceive").

The Latin sense re-emerged in English as "prudence in regard to danger" (1650s). Meaning "word of warning, monitory advice" is from c. 1600. Meaning "anything which excites alarm or astonishment" is U.S. slang, 1835.

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cautionary (adj.)
"conveying a warning," 1590s, from caution (n.) + -ary.
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cautious (adj.)

"careful to avoid danger or misfortune," 1640s, from caution + -ous. The Latin word for this was cautus "careful, heedful." Related: Cautiously; cautiousness.

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precaution (n.)

"previous caution, prudent foresight (to prevent mischief or secure good results); a measure taken beforehand, an act of foresight," c. 1600, from French précaution (16c.) and directly from Late Latin praecautionem (nominative praecautio) "a safeguarding," noun of action from past-participle stem of Latin praecavere "to guard against beforehand," from prae "before" (see pre-) + cavere "to be one's own guard" (see caution (n.)). In mid-20c. a euphemism for "contraception." The verb meaning "to warn (someone) in advance" is from c. 1700.

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*keu- 

also *skeu- Proto-Indo-European root meaning "to see, observe, perceive." 

It forms all or part of: Anschauung; caution; cautious; caveat; kudos; precaution; scavenger; scone; sheen; show.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit kavih "wise, sage; seer, poet;" Avestan kauui- "seer, poet, wise man;" Middle Iranian škoh "splendor, majesty;" Latin cautio "care, foresight," cautus "careful, heedful," cavere "beware, take heed;" Greek kydos "glory, fame;" Lithuanian kavoti "tend, safeguard;" Armenian cucanem "I show;" Old Church Slavonic čudo "wonder;" Czech (z)koumati "to perceive, be aware of;" Serbian čuvati "watch, heed;" Old English sceawian "to look at," Middle Dutch schoon "beautiful, bright," properly "showy," Old High German scouwon "to watch."

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precautious (adj.)

"using precaution, displaying previous care or caution," "1680s, from precaution + -ous. Related: Precautiously; precautiousness.

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carefulness (n.)
Old English carfulnys "anxiety, solicitude;" see careful + -ness. Meaning "heedfulness, caution" is in late Old English.
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foresight (n.)
also fore-sight, early 14c., "insight obtained beforehand;" also "prudence," from fore- + sight (n.). Perhaps modeled on Latin providentia. Compare German Vorsicht "attention, caution, cautiousness."
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caveat (n.)

"warning, hint of caution," 1550s, Latin, literally "let him beware," 3rd person singular present subjunctive of cavere "to beware, take heed, watch, guard against," from PIE root *keu- "to see, observe, perceive." Legal sense "public warning preventing some action" is from 1650s.

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