Where did the idea of manifest destiny come from and why?

It began in the 19th Century.  It means that the Americans believe that it was their destiny to expand themselves throughout the American continent.  As the U.S. gained more land, they moved in and established settlements and eventually gained statehood.  Later they also used this when they began a campaign of imperialism.

the chair is the committee member from the majority party who has the most seniority. in the south carolina senate, the chair is the committee member from the majority party who has the most committee seniority.