Thinking over the change of ain't (am not) to ain't (is not); and prompted
by the posting from Jean Murphy; I offer for discussion the whole field of
word meaning changes.   The first that comes to mind is 'nice' from 'fussy
to pleasant'.   Another that comes to mind is 'sophisticated', meaning
'highly cultured' rather than 'complicated'.   My third example is 'bland',
from 'mild to dull', which is I suppose something like 'the grand old Duke
of York', neither up nor down.
 
Donald Hocking
Procedural & Protocol Consultant
<http://www.ozemail.com.au/~donaldh/art0.htm>