RE> Lance Cousins has a very thorough discussion of this term in his article
RE> "Good or Skilful? Kusala in Canon and Commentary", published in the online
RE> Journal of Buddhist Ethics: http://jbe.gold.ac.uk/3/cousins1.html

And here is a conclusion of this article with which I largely agree:

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Semantic development

Returning to ku"sala, the semantic evolution I see is:

- An original meaning of "intelligent" or "wise";

- Expert in magical and sacrificial ritual (in the Braahma.nas); for
brahmins, of course, this would precisely constitute wisdom.

-- Skilled in meditational/mystical (/ascetic?) practices (in the
early Pali sources and, no doubt, in other contemporary
traditions), including skilled in the kind of behaviour which
supports meditation, etc. i.e. siila, etc.

-- Skilled in performing daana and ya~n~na, now interpreted in
terms of Buddhist ethical concerns; and associated with keeping the
precepts and so on.

- Kusala in later Buddhist and Jain sources becomes generalized to
refer to something like wholesome or good states.

So there is no reason to doubt that by a later period (i.e. in the
commentaries and perhaps later canonical sources) kusala in
non-technical contexts meant something which could be translated as
"good".

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So in early Pali texts 'kusala' mostly means 'skilful', and in later
'good'.

Best Regards,
Dimitry