"chief commander of a regiment of troops," 1540s, coronell, from French coronel (16c.), modified by dissimilation from Italian colonnella "commander of the 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").
The French spelling was reformed late 16c. English spelling was modified 1580s in learned writing to conform with the Italian form (via translations of Italian military manuals), and pronunciations with "r" and "l" coexisted until c. 1650, but the earlier pronunciation prevailed. Spanish and Portuguese coronel, from Italian, show similar evolution by dissimilation and perhaps by influence of corona. Abbreviation col. is attested by 1707.