Why is the ninth month called September?
c.1300, "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" (see caveat). The Latin sense re-emerged in English 16c.-17c. Meaning "word of warning" is from c.1600.
"to warn," 1640s, from caution (n.). Related: Cautioned; cautioning.