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.