Unknown. Generally considered to be from a Vulgar Latin *excubulāre (a term used in hunting, meaning to remove from an animal's den or nest), but this is a difficult link to make semantically and phonetically. Another possibility is a link to a Latin root *collāre, from Ancient Greek (compare Italian collare, scollare), or less likely, succollāre. Overall, it is probably ultimately tied to *collum, from Latin collis.