Boswell tells us that Samuel Johnson made this famous
pronouncement that patriotism is the last refuge of a scoundrel
on the evening of April 7, 1775. He doesn't provide any context
for how the remark arose, so we don't really know for sure
what was on Johnson's mind at the time.
However, Boswell assures us that Johnson was not indicting
patriotism in general, only false patriotism.
For more of Samuel Johnson's thoughts on patriotism in
general, go to the patriotism
For a discussion on a possible false patriot who
Johnson might have been thinking of, see this discussion.