No established opinion on the origin. On the one hand, it may be considered as a by-form of the well-known PWS
*khl with the same meaning (cf., e.g., CDG 277, HALOT 1891 and Huehnergard–Olyan
2013); on the other hand, one might venture a direct comparison
with PS
*wkl ‘to (en)trust’. While it is certainly true that “‘to trust’ is not semantically
close to ‘to be able’” (Huehnergard–Olyan 2013:13), the gap is probably not so drastic either
(“to have trust” > “to have power to do something” > “to be able”).