means The learned ability to do something well, built up through practice and experience.

from From Old Norse 'skil,' meaning a distinction, discernment, or knowledge of how to tell things apart. The word arrived with Norse settlers in England and originally meant 'discernment' or 'reason' — the sense of competent doing came later, as if to say that knowing the difference between right and wrong ways of acting is itself the root of doing anything well. It's possibly related to the verb 'shall,' through an old shared notion of dividing and arranging.