Why is the ninth month called September?
late 14c., "to settle a dispute," from Old French decider, from Latin decidere "to decide, determine," literally "to cut off," from de- "off" (see de-) + caedere "to cut" (see -cide). For Latin vowel change, see acquisition. Sense is of resolving difficulties "at a stroke." Meaning "to make up one's mind" is attested from 1830. Related: Decided; deciding.