colonel (n.) Look up colonel at Dictionary.com
1540s, coronell, from Middle French coronel (16c.), modified by dissimilation from Italian colonnella "commander of a column of soldiers at the head of a regiment," from compagna colonella "little column company," from Latin columna "pillar," collateral form of columen "top, summit," from PIE root *kel- (2) "to be prominent; hill."

English spelling modified 1580s in learned writing to conform with the Italian form (via translations of Italian military manuals), and pronunciations with "r" and "l" coexisted 17c.-18c., but the earlier pronunciation prevailed. Spanish coronel, from Italian, shows a similar evolution by dissimilation.